So I decided to combine them both into one, and write them out here today.
The purpose of these posts, is to inform the lay person, who is interested in either learning about or writing about guns; either in a non-fiction context (news reporting for example), or in fiction.
The problem is, most good writers don't know a lot about guns (there are a few exceptions, like Stephen Hunter for example) ; but a lot of them do write in genres that a lot of "gun people" like to read (military and adventure fiction, science fiction, thrillers and mysteries etc...).
The mistakes these authors and writers make are quite jarring to those of us who list guns as a hobby, a passion, or a profession; and they can ruin our enjoyment of the work. And for those who don't know, and aren't interested in guns, it spreads unrealistic, or even silly, stupid, or dangerous, misconceptions about guns.
Importantly, I don't just believe, I know for an absolute certainty; that silly stupid inaccuracies about guns in pop culture, are a large portion of the cause of the silly stupid laws and regulations made around guns; said laws and regulations mostly written by, and voted for by, people who don't know any better.
My hope is, that with posts like these, a simple internet search can help out someone who doesn't understand, and is willing to do some basic research.
For the non gunnnies, I am going to try to make this as clear as possible. I'm going to be pumping out a LOT of information here, and some of it is going to be mathy, and technical, and gun geeky. I'm trying to minimize it, but it's unavoidable; just be assured that if I mention it here, it's something you should probably know, or which will at least be helpful for you to know.
For my readers who are gun hobbyists, I will caution you, I am DELIBERATELY simplifying this stuff.
Yes, some of what I will be saying in this post is generalization, estimation, rounding off etc... Don't blow a gasket because I don't reference the exact mass and velocity of a particular cartridge etc... The people I'm writing this for don't want, or need, to know that extreme detail. What they need to understand is the general concept, enough detail to not make dumb mistakes, and enough specifics to understand WHY (because if you don't understand why something is what it is, you don't know when you're making a mistake). Trying to go deeper than that will just make the whole thing more difficult to read, and serve no real purpose except wanking.
Ok, moving on to the meat of the issue...
So, the question in question (forgive me, I couldn't resist) is VERY common... and in fact I'm surprised it hasn't come up in a way that made me write about it directly, some time in the last 7 years.
I've heard this question a million times, in a million different ways, but yesterday it was phrased thusly:
"What is the biggest, most powerful gun? Can you please tell me the specific make and model"
Whoa boy...
Well...
Now, my readers who know guns probably all just said to themselves something like "well, that's an impossible question to answer"... and as phrased it is.
There is no one "most powerful gun"...
And that is true for a number of reasons.
First, you have to make a distinction between "largest" and "most powerful". They're two very different things. Generally bigger guns are "more powerful" than smaller, but not always, and not necessarily in a consistent or even logical manner.
Then, in "biggest" or "largest", there's no clear answer, because there's a "largest caliber", there's a "heaviest projectile", there's a "longest projectile", there's a "biggest cartridge case", and other factors.
"Most powerful" is very hard to define as well, because you've got both mass, and velocity to account for; and the standard objective power metric of "muzzle energy", isn't necessarily a good predictor of combat, defensive, or hunting effectiveness.
Also in the "most powerful" question is the weighting of multiple rounds on target. A bolt action .50bmg rifle is capable of doing a lot of damage with a single round; but a machine gun in .308 can put 100 rounds downrange before the .50 can put two... and that is certainly more gross damage potential, but is it "more powerful" ?
I think, just to simplify things, we need to exclude machine guns, multibarrel rotary guns etc... from the equation, and just talk about the size and power of a single fired round. Also, we need to exclude things like exploding ammunition, recoilless rifles, rifle grenades etc...
Actually... even before all that, we have to limit ourself somewhat further, and assume that we are talking about "small arms" fireable by a single person without mechanical support; which is what most people mean when they say "guns".So, moving on from that, the question is still basically impossible to answer simply, because the "power" of a firearm is determined not by make and model, or actually anything about the firearm itself; but by the ammunition that it fires.
If you want to include all things defined as "guns", the "most powerful" is going to be either the Gerald Bull "supergun", which though it never fired, would have been about 150 feet long and 16 inches in diameter, but which could fire a projectile into the edge of space; or an artillery piece launching a nuclear weapon. For "biggest" you would probably choose one of the absolutely gigantic 24" to 36" artillery pieces created during WW1 or WW2, which launched projectiles as much as 12 feet long and weighing as much as 15,000lbs.
Specifically, the power of a firearm is determined by both its chambering (the designation of the cartridge, generally signifying it's diameter and overall length), and the specific loading of that chambering (the amount and type of powder used in the cartridge, and the weight of the bullet used, both of which can variables in a single chambering).
In general, there are many different weapons, from many different manufacturers, available in most common chamberings. For example, pretty much every major handgun manufacturer makes guns in 9x19 nato (commonly just called 9mm), and .45acp.
As of 2012, these two chamberings are the most commonly available centerfire pistol chamberings in the U.S. ; though both chamberings were originally available in a single firearm, and have been strongly associated with those firearms, such that some people still refer to them as "9mm luger" and ".45 colt" (though both are inaccurate, and the second is ambiguous, because it's also commonly used for a large .45 caliber revolver chambering, developed about 40 years before the .45acp).
Also, in general, there are many different loadings (as I said above, that means different bullet weights, and different powder charges), at many different power levels, for most chamberings.
Further, many people hand load their own ammunition, to be more powerful (or more precise) than factory offerings (which is quite easy to do. Factory ammunition is generally quite conservatively loaded, to avoid product liability; as it may be unsafe to load ammunition "hot" - meaning to a higher power level - in older, or less well made guns).
There are only a few, generally very uncommon (or custom, called "wildcat", and usually not available from a factory other than that of its inventor/developer), chamberings, that are only chambered by one particular make and model of gun.
Finally, there are very big differences in the size, and the power, of revolvers, semi-automatic pistols, shotguns, and rifles; so much so that they are not directly compatible, and need to be taken individually as separate questions.
A few technical notes on ammunition, terminology, math, and physics:
Remember at the beginning of this post where I said I realized that the question asked required me to actually write two posts... well this is that other post.
In the second part post I am going to be talking about the mass, velocity, and energy of projectiles launched from firearms; as well as some details about ammunition.
In discussing these subjects, there are certain specialized conventions in terms and measurements which are unique to the firearms world; as well as some certain basic knowledge about ammunition that is necessary to understand the overall discussion.
I could have split this into two posts, but since I think it's more useful and easier to understand things in context, I decided just to make one very long post.
Sorry.
Ok, so what do you "need to know" about ammunition?
The first thing to note is that individual pieces of ammuniton are not "bullets", though they are often referred to as such; they are properly called either cartridges (for rifles and handguns) or shells (for shotguns).
Many people not familiar with firearms... and even those who are, and are just using it as shorthand (hell, I do it myself fairly commonly)... refer to an entire cartridge as a "bullet". This is technically incorrect, though common usage; and gives rise to the common misconception among the unfamiliar that the entire cartridge is fired from the gun.
The second thing to understand, is the parts of a cartridge or shell (as I have reviewed in detail in other posts; but I'll go over them here again for clarity).
A centerfire or rimfire cartridge has four basic parts, and a shotgun shell has five (which are slightly different than a cartridges parts):
The Bullet: This is the actual projectile that is launched from the firearm. When looking at a cartridge, the bullet is the rounded or pointy bit at the end; usually made of lead, or lead and copper (but sometimes made of other materials such as brass).
An important note: small arms bullets are almost never made of steel; though they may have some steel in them. The term "full metal jacket" (abbreviated as FMJ) is common, and correct; but usually refers to a copper, cupronickel alloy, or brass jacket, not steel. Many people believe that it means the bullets are made of steel, or jacketed with steel, but that is not generally true (though as I said, it sometimes is; notably a lot of eastern European ammo is either steel or steel alloy jacketed, as copper is considerably more expensive, and historically was more difficult to obtain in communist countries). some ammunition may also have a steel core inside the copper and lead, which increases penetration through hard surfaces.
The Case: This is the part of the cartridge that holds the bullet (and the next two parts the propellant charge and the primer). It is generally made of brass, but can also be made of steel or aluminum; or brass plated with nickel. Generally speaking, the case is far longer and and far heavier than the bullet (or in fact, heavier than all the other pieces put together), and may be far larger diameter (frequently they are, in rifles, but rarely in hand guns). It is also often called "the casing", or "the brass".
The case itself has a few parts: The mouth, which is the open end that the bullet is inserted into, and in the case of "bottleneck" cartridges, the neck and shoulder; the walls or body, which are the sides of the case that hold the powder charge, and bear against the firearms chamber (the part of the gun that actually holds the cartridge while it is being fired. It's at the end of the barrel closest to the magazine of the gun); and the head (which includes the primer pocket, and the extractor groove or rim) which is thicker and heavier than the rest of the case to accept the primer, to handle the pressure of firing, and to handle the mechanical stresses of feeding the cartridge and extracting the case after firing.
Cartridge cases are generally marked on their case heads, with the manufacturer of the case, and the chambering the case is designed for. This is called the "headstamp".
One should note that headstamps are generally accurate and correct for factory produced ammunition; but many people hand load their own ammunition, in custom chamberings, using brass that was originally designed for other chamberings, and as such the actual chambering of a fired case may be different from the headstamp.The Propellant Charge: Also called the powder charge, the charge, or the powder; this is the actual propellant, which is ignited to fire the bullet out of the barrel.
Note, I did not say "exploded" or "explosive" I said "ignited" and "propellant", which is an important difference. The powder charge of a modern cartridge doesn't actually explode, it just burns very very quickly in what is called "deflagration". There are many different formulations of modern smokeless gun powder; all of which have different burning characteristics, and produce different levels of energy.
Modern gunpowder is considered "smokeless". Though there is some smoke from modern smokeless powders, it is very little in comparison to traditional "black" powder, which is chemically very different.
Black powder actually does explode or rather detonate, rather than just rapidly burning; and it produces an enormous amount of fairly foul smelling smoke as it does so.
Black powder is composed of sulfur, potassium nitrate, and charcoal which is combined into a wet slurry, then allowed to harden and ground down to a fine "kern" or "corn" (which is given a letter grade to denote fineness). Modern smokeless powder is composed of cellulose (plant fiber) which has been dissolved in nitrating compounds (usually nitric acid, or nitric acid and nitroglycerin), mixed with various additives, then extruded and dried into small cylindrical grains, balls, or flakes.
VERY IMPORTANT FOR WRITERS... Modern smokeless powder IS NOT CORDITE.
Many authors who don't know better have read the phrase "the tang of cordite in the air", and then reused it; almost all of them incorrectly and improperly.
Cordite was a specific type of smokeless propellant invented by the British in the late 1880s, used mostly by them (and their colonies and commonwealth members), and only commonly used in small arms ammunition prior to the 1920s (though artillery shells sometimes used cordite up through the 1950s). It was called cordite, because it wasn't actually a powder; it was actually extruded in long yarnlike strings or "cords".
As for the "tang of cordite", the stuff did have a very distinctive aroma of ammonia and potassium when ignited, but the amount ignited in a rifle cartridge is so small that the smell is quite faint (and yes I have fired 90 year old cordite ammunition. You can still buy the stuff that was made for world war one; though it's not particularly reliable, and it's horribly corrosive). It takes the large scale firing of machine guns, or hundreds of rifles, or of an artillery piece (which has the same amount of propellant as hundreds of rifle cartridges), to "taste the tang of cordite in the air".
Black powder was used in the western world from the 14th century, until the early 20th century when it was replaced (except in recreational and "traditional" hunting use) by smokeless powders. The transition from black powder, to smokeless powder, began in the last two decade of the 19th century, and was complete prior to world war 1. These days, people only shoot blackpowder for fun, and because many states have a hunting season limited to blackpowder firearms.
The Primer: Smokeless powder isn't impact sensitive (or at least not significantly so). It isn't set off by the firearms hammer slamming into it; it needs an impact sensitive explosive to ignite it. That is the function of the primer.
The primer is a small metal cup, with a tiny bit of impact sensitive explosive sandwiched into it in between between a couple pieces of thin metal, shown here:
The primer is inserted in the head of the cartridge, which has a tiny hole drilled into it (called the flashhole). Shown here is a primer pocket and flash hole, as well as an expended primer in the case head (both showing the head stamps I mentioned above):
The primer is hit by the firing pin or the striker, which causes the impact sensitive explosive to detonate; and a tiny jet of flame shoots up through the flash hole, igniting the powder charge, and propelling the bullet down the barrel and out the muzzle of the gun (along with a lot of expanding gas, and unburned powder and other residues of gunfire, as so often described on CSI).
Ok, what was that about shotgun ammo?
Shotgun shells are a little bit different. They have five parts, which serve mostly the same functions as in cartridges, but are made a little differently.
Shotgun shells have a powder charge and primer that are largely the same as centerfire cartridges, but there are three basic differences:
The Hull: This is analogous to the case in handguns and rifles, but is slightly different. Modern shotgun hulls are generally made of plastic with a brass base, or "head"; though historically (and currently used in shooting events that recreate historic time periods) they have been made of paper and brass, or even all brass, like a handgun casing.
Also, rather than being capped off with a bullet, shotgun hulls have either a crimp, or a sealing disk of cardboard (crimp shown here as the kinda star shaped thing):
The Load: Shotguns dont fire bullets, they fire either shot, or slugs, and whatever is used as a projectile or projectile is called the load. Shot, is a number of small balls anywhere from as small as 1mm to as large as 9mm across, usually made of lead, but sometimes made of copper, bismuth, tungsten, or steel; and each shell could have as few as five or six, to as many as a couple hundred individual shot pieces (depending on the size of shot, and weight of the load). Slugs, are single large copper, brass, or lead projectiles, which look more like badminton shuttlecocks or salt shakers, than they do bullet; but basically they are just a REALLY big, soft, bullet.
Here are a couple different types of slugs:
And here's a cutaway showing both shot and slugs (as well as the rest of the components):
The Wad (or wad and Cup): In handgun and rifle cartridges, there is nothing between the bullet and the propellant, but air. With shotguns, because they often fire large numbers of small pellets, in order to keep the load all together moving down the barrel, there needs to be a solid buffer between the propellant charge and the shot load. That solid buffer was once almost always a few disks of cardboard or heavy paper (and still sometimes is), but for decades now it has frequently has been a thin plastic cup with a thick plastic base. It flies out the end of the barrel surrounding the load, but because it is very low mass, and has a high surface area, it separates from the load within a few yards, falling to the ground (though sometimes if you are close enough to a paper target, the wad will go through the target).
Here's a picture of an actual wad:
Here's another cutaway diagram showing the wad and cup a bit more clearly:
Now, some more of the terminology:
When we're talking about firearms "power", we need to talk about mass, velocity, and energy. There are lots of terms used in science for these things. In science, mass is generally denominated in grams, velocity in meters per second, and energy in joules for example.
In Europe they sometimes use those terms, but ballistics has been around a lot longer than the metric system, and in the U.S, we generally use the traditional terminology.
The actual caliber of a bullet is just the diameter of the bullet, expressed either in inches (or more commonly in fractions of an inch, as almost all small arms fire projectiles smaller than 1 inch in diameter) such as .45 caliber, or in millimeters, such as 9mm.
It is technically accurate to say something is in "9mm caliber", however it is not commonly done, as in general, when someone hears "caliber", they are expecting to hear an inch measurement.
All that said however, the bullets diameter isn't directly related to the "power" of the firearm; except in that larger diameter bullets tend to be heavier (have more mass), than smaller diameter bullets (though not always, as bullets can be short and stubby, or long and thin).
...Well that, and that bigger bullets tend to make bigger holes in whatever they hit; and bigger holes usually hurt more...
Bullets mass and powder charge are both denominated in grains, abbreviated as gr.
1gr is 1/7000th of a pound, or 437.5gr per ounce.
Note that gr=grains not g=grams.
So, a standard 230gr .45acp bullet weighs a bit more than 1/2 ounce; and a standard 9mm bullet weighs a bit more than 1/4 ounce. What generally surprises folks who aren't familiar with guns, is that the bullet for our standard military rifle, which is chambered in 5.56 nato, is even smaller; at just under 1/4" diameter, and weighing in at right around 1/8 of an ounce.
Smaller... but much more energetic; because kinetic energy is a function of both mass, and velocity. More importantly, energy scales linearly with mass, but exponentially with velocity (so velocity has a much greater weight in the equation as it goes up).
In American practice the custom is to denominate muzzle velocity in feet per second, and energy in foot pounds force, usually just referred to as foot pounds, and marked as ftlbs or ft/lbs.
For those of you using the metric system, 1.35 joules is approximately equal to 1 foot pounds force, and 1 meter per second is approximately equal to 3.28 feet per second.
Now, the physics and the math...
The only really objective and quantifiable measure of firearms "power", is kinetic energy; which is conventionally called "muzzle energy" in ballistics.
Kinetic energy is calculated by the formula K=(1/2)MV^2, however as in any other equation, the units must be dimensionally comparable (remember dimensional analysis from high school?).
Thus, the equations work out as E=M*V^2/450400 for the American system and E=MV^2/2000 for the metric system.
M is bullet mass (denominated in either in grains or grams). V is the bullet velocity (in either ft/sec or m/sec) The constants 450400 for american measurement, and 2000 for metric measurements, are the conversion factors for the dimensional analysis.
Here's a link for a calculator to do the math for you, which prints a nice little graph of total kinetic energy as a function of mass and velocity.
So, a 230gr .45acp bullet travelling at the standard 830 feet per second, has about 350ftlbs of muzzle energy; and a 62gr 5.56 nato bullet (which is the same as a .223 Remington, about half the diameter of the .45) travelling at the standard 3100ft/sec, has about 1300flbs of energy.
That's about four times the energy in the rifle round vs. the pistol round; even though the rifle bullet is only about 1/2 the diameter, and about 1/4 of the mass.
Physics is FUN YAY!!!!
So, all that said, let's make some basic assumptions and generalizations, and talk about some of the most powerful common chamberings, and some of the guns that chamber them.
HANDGUN POWER IN GENERAL:
Both the largest and most powerful handgun chambering is the Smith and Wesson .500 magnum.
It is a true .50 caliber pistol round (which is the largest caliber generally allowed under U.S. sporting arms regulations), with the same muzzle energy as a large magnum rifle. It fires up to a 500gr bullet at up to 1600feet per second for around 3000ftlbs of energy.
As of now, it's only available in the Smith and Wesson x-frame revolver, and a few custom revolvers; mostly because the cartridge itself is both too large (both long and wide) and too powerful, for most other handgun frames.
The most powerful commonly available chambering that fits into a "normal" sized revolver is generally considered to be the .454 casull; which is a .45 caliber round, at up to 400gr, and up to 1500feet per second for up to around 2000ftlbs of energy.
The most powerful chambering that people would actually carry for self defense, is the .44 mganum (.454 is used for hunting, and protecting yourself against bears), or a heavy .45 colt loading; and in general most still would consider those too hot for self defense, and use them generally for hunting.
The most powerful chambering that is commonly carried for self defense in revolvers is .357 magnum. Very heavy, very hot hunting rounds can be had with bullets up to 180gr and at velocities of up to 1500fps, for muzzle energies well over 800ftlbs; but that would be VERY hot ammo, and not safe for firing in most normal guns. The most powerful loads people would actually carry, are somewhere around 160gr at around 1500fps, for around 800ftlbs.
The great thing about .357 magnum revolvers, is that they can shoot the much less powerful (and much cheaper) .38 special ammunition (.357 magnum is basically stretched and overpowered .38spl in a tougher brass case). They can also be made quite small. I have a pocket sized 5 shot .357 magnum revolver that only weighs 10 ounces empty, and about 12 ounces loaded. It goes into my front pants pocket in the morning and doesn't come out until I go to bed; and I barely notice it at all.
semi-automatic pistols are, in general, not able to handle the same power levels as heavy, large framed revolvers, for a number of reasons relating to how semi-autos function. You CAN get very powerful semiautos, but they have to be very large, and very heavy, like the Desert Eagle or the Wildey.
The most powerful commonly available chambering in a semi-automatic pistol is the .50 action express; but it's only available in the Desert Eagle, and some custom guns. It shoots up to a 350gr bullet at up to 1500fps, for up to around 1600ftlps of energy.
There are also a few boutique chamberings like the .45 winchester magnum, the .45 wildey magnum, the .44 automag etc... etc... some of which are as powerful as moderate loadings of the .454 casull, but most of which are under the hottest loadings of the .44 magnum.
The most powerful chambering in semiautomatic pistols people actually CARRY, is probably 10mm (I often carry one myself); which fires up to a 180gr .40 caliber bullet, at up to 1350fps, for around 730ftlbs of energy; somewhere in between the .357 magnum and the .44 magnum revolver rounds (about the same as the much less common .41 magnum).
There are a number of pistols that can chamber rifle rounds; which are generally far more powerful than pistol rounds; but again, those are all rather specialized firearms.
SHOTGUN POWER IN GENERAL:
The most powerful shotgun commonly available is the 10ga, but shotgun "power" is highly dependent on the ammunition you are firing... As it is for all other firearms certainly, but more so for shotguns.
Shotgun loadings are far more variable than handgun or rifle loads, generally because shotguns are used for a broader range of activities than other firearms; and also because shotguns will properly and accurately function with a broader range of ammunition, while rifles and handguns (and semi-automatic shotguns for that matter) need to operate in a fairly narrow range of mass and velocity to function both reliably and accurately.
These shotgun loads can vary from light trap loads, which generally use a shorter 2-1/2" or 2-3/4" shell (or even 1-3/4" in some specialty loads), and launch relatively light loads of very small pellets at a moderate velocity to hit clay targets; to heavy 3.5" magnum slug loads, which launch a single very heavy projectile, at relatively high velocities, for hunting and personal defense.
In general, you would probably categorize the most powerful ammunition options as 3.5" magnum slugs; which are available in 10ga, and 12ga.
A not on shotgun terminology; as with wire gauge, the smaller the gauge, the larger the diameter of the bore size. Shotgun gauge is also sometimes referred to as "bore", which was a measurement dating back to old blackpowder cannons. In this context refers not directly to diameter, but to the number of pure lead spheres of the diameter of the barrel that would fit into a pound of lead. So a 12 bore or 12 gauge shotgun has a bore diameter equal to that of a sphere of lead that weighs 1/12th pound; which corresponds to about .72". A 10ga that of a 1/10th pound lead sphere which is about .77" and so on.
A 10ga 3.5" magnum slug shell fires a .75" diameter slug, weighing approximately 2 ounces (I have seen a 2-1/4" oz slug load, but they are very uncommon. The heaviest common slug load would be 1-3/4oz, at approximately 1300 feet per second, for somewehere around 3000ftlbs of energy (which is about the same as the .500 S&W).
Oh I should have noted, the 2 ounce 10ga slug I mention above is about 875 grains; but traditionally, shotgun loads are denominated in ounces.
RIFLE POWER IN GENERAL:
In rifle terms... there's really no answer to that question, as there are far too many variables.
HOWEVER, IN GENERAL...
The most powerful commonly available chambering is the .50bmg; a round originally designed in 1917 for heavy machineguns, and still used for that today.
There are hundreds of different rifles available in .50bmg; from around $1000, to over $20,000; including both bolt action and semi-automatic options.
It's the largest commonly available, because U.S. sporting arms regulations generally limit modern smokeless powder cartridge firing rifles to .50 caliber or below (there are a few exceptions), and also because more powerful weapons generally generate too much recoil and muzzle blast to be fired from the shoulder, and need to be mounted into the ground or on a vehicle.
There ARE a few rifles chambered in 20mm (which is about .79 caliber, and usually using aircraft cannon type ammunition with a solid projectile instead of an explosive shell) but they are very rare, very expensive, and very very hard to shoot; because of the massive recoil energy and muzzle blast they produce. They are essentially a curiosity.
The use of the .50bmg round in accurate bolt action and semi-automatic rifles however is fairly common these days. It has a fairly long history dating back at least to the 1950's (I'm sure people built .50 caliber bolt action rifles before then, but I can't find any references to it), and could now be considered on the edge of mainstream.
The .50bmg can be accurate at over a mile under the right conditions; and has made kills at far longer ranges (sniper kills at 1.5 miles; machine gun kills from indirect fire at over 2 miles).
The standard load for a .50bmg is an 800 grain bullet, that's .50" wide and over 2" long, at around 3000 feet per second, for around 15,000 ftlbs of energy.
That's about 6 times the power of a common large battle rifle chambering like .308 or .30-06; and about 12 times the power of the standard 5.56 nato assault rifle chambering.
Of course, it's also got about 12 times the recoil (actually, it's about 8 times the recoil impulse, and 11 times the total energy. Here's a calculator to help you determine recoil if you're interested) and needs to be fired in guns that weigh at least 4 or 5 times as much etc... etc...
The most powerful chambering commonly used by both hunters and soldiers, is the .300 winchester magnum, which fires a bullet between 160gr and 220gr at somewhere arounf 3000 feet per second, and somewhere around 4,000 ftlbs of energy.
The .300 winmag is used by snipers, and by long range hunters, as well as long range shooting competitors (my 1000 yard rifles are all in .300 win mag). It's very commonly available, with ammunition and rifles chambered in it both available at your local Wal-Mart in most states.
The .338 lapua is a chambering that has become more common over the past 15 years or so. It's also used by snipers and long range shooters, but not much by hunters; as it doesn't give much advantage over the .300 winmag until you go beyond 1000 yards range. It's got about 50% more energy than the .300, and is good out to past 1500 yards, with a few confirmed kills past 2000 yards; for a HUGE recoil, noise, and cost penalty (almost as much as a .50bmg when you consider it's being shot in rifles that weigh half as much as the .50s do).
There are other very large, very powerful rifle rounds sometimes used by snipers or other long range shooters, often based off of .50bmg cases, but sometimes based off of large magnum rifle rounds; but I wouldn't call any of them common, and none are actually more powerful than the .50bmg.
There ARE however rifles that fire larger diameter bullets, and even heavier bullets; they just fire them at much lower velocities.
These chamberings are the classic african "elephant gun" loadings; which went up as high as .70 caliber (the .700 nitro express), using bullets massing as much as 1000gr, and producing as much as 10,000ftlbs of energy.
Unlike sniper and other long range rifle rounds, these big monsters were designed to shoot at short ranges of under 100 yards (generally 50 yards or less in fact); and to deliver their HUGE bullets, at moderate velocities, to break through the heavy bone structure of African big game like Elephants, Rhinos, and Cape Buffalo.
There have even been a very small number of even larger rifles made; which shoot bullets so large they are referred to by "bore" rather than caliber, much as shotguns are (shotgun gauge and shotgun bore are technically interchangeable, but "bore" is archaic... though still commonly used among older shooters, and the english).
There have been 8 bore, 4 bore, and even 2 bore rifles; again used by african hunters, and firing projectiles spanning as much as a 1.32 inches diameter, weighing as much as 8 ounces, and producing as much or more energy (from about 1/2 ounce of black powder, which is generally less energetic than modern powders) as a .50bmg (though at much lower velocities of course)
I have heard that a few folks have, in recent years, produced 1 bore rifles as stunts; but I don't think anyone ever actually made them to go after game in Africa... In fact, with a 1 pound projectile at around 1200 feet per second, producing 23,000 ftlbs of energy; when fired from a 20lb gun, I'm pretty sure you couldn't fire one from the shoulder without actually doing damage to yourself. The recoil energy and impulse of such a weapon would be enough to tear your shoulder out of its socket.
These rifles are very rare, very expensive, and very specialized. They are not in common use by any stretch of the imagination; but they are out there.
So then, what IS the biggest, most powerful gun?
Well, given all that I've just said, and since there is no one answer... why don't you tell me?
Like I said, it depends on your definitions of "largest", "most powerful", and "gun".
Hopefully, 6000 words later; I've given you enough information here that, in the context of what you are trying to write, you can figure those out for yourself.
