Tonight, I'm back once again to address a common firearms myth; this one actually a bit more technical than most. We're going to talk about bullet stabilization, specifically that of 5.56 nato and .223 bullets.
This is a common subject of misinformation, because most folks don't understand what bullet stabilization is, or how rifling really works for that matter.
Worse, 5.56 is the default chambering of the AR platform rifle, which is the default rifle choice for tactical Tommies everywhere; and as we all know mall ninjas love nothing more than incorrect information.
Actually, to be fair, even otherwise knowledgeable gunnies don't generally understand the variables involved in stabilization, because they don't have a background in the physics or aerodynamics of external ballistics; and because they hear a lot of bull, that sounds kinda OK, and since they don't know any better, take it as truth.
Before we even begin, let me refer you to the best source on ammunition for the AR platform, and 5.56 nato in general; the ammo oracle. There really is no better collection of information on 5.56 ammunition anywhere.
Also, these same principles apply to all elongated bullets (i.e. anything that isn't a ball), no matter what the caliber is; I'm just using the 5.56 as an example, because it is the most common rifle available with many different twist rates, and also the chambering for which the most misinformation is circulating.
Ok, down to business
First, what is stabilization and how does it work?
Modern rifling has a twist, to impart spin to the bullets leaving the barrel. This spin helps to make bullets in flight more stable in two ways:
First, the spin causes gyroscopic stabilization; which is just like how it sounds. A spinning mass resists being disturbed off the axis perpendicular to the direction of rotation due to gyroscopic inertia; which is rigidity in space induced by radially symmetric (which means the forces are the same along all radii - i.e. identical all the way around in all directions) centrifugal forces.
This is the primary component of spin stabilization for pointed bullets (ball bullets are primarily stabilized aerodynamically, because they tumble as well as spin), and it's force component significantly outweighs most of the aerodynamic components of ballistic stability which I will describe in the next section.
In simple English, bullets spin like a top, and they don't "fall over"; just like a top doesn't.
Now, the second element of stabilization caused by rotation, is aerodynamic. Objects rotating in a fluid (and that's what air is), generate radially symmetric lift (again, this means the lift is the same in all directions).
Anyway, this means the bullet is neither pulled down, nor pulled up by these forces (technically this is incorrect in some very small and specific ways, but for purposes of this illustration it is a valid assumption); or rather it is pulled down and pulled up, as well as pulled to all sides and in all radial directions, equally. When something is being pulled in all radial directions equally, just as with a gyroscope, it resists deflection in the axis perpendicular to the pulling forces.
There are two other components of aerodynamic stabilization, and those are form lift (which is the lift created by the shape of the object itself as it passes through the air) and the angles of attack and incidence; but neither are useful in this discussion at the moment.
The first common myth about stabilization, is that the heavier a bullet is, the faster it must spin to be stabilized. In fact this isn't really true, heavier objects gyroscopically stablize at lower rotational velocities than lighter objects (the flywheel effect); and though the aerodynamic stabilization componenst required for heavier objects are greater in magnitude than for lighter objects, the differences in weight and relative difference between force components, between different examples of the same diameter bullet loaded for the same cartridge, are generally small enough that the aerodynamic component of the stabilizing forces required do not change significantly.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
The reason why length is important, is because of the center of gravity, and center of pressure of the bullet; and their relationship.
Center of gravity is a commonly known (if not necessarily commonly understood) concept; which simplified, is the balance point of the bullet. If you very carefully put the bullet on a razors edge at the exact center of gravity, it would in theory balance and just sit there, stable.
The center of pressure is a similar concept; in that it is the point where the aerodynamic forces acting on the bullet are balanced along the longitudinal axis of the bullet (the length).
If your center of gravity, and center of pressure are identical, then your bullet will exhibit exactly neutral stability. This means that the bullet will neither resist deflection, nor will it correct or accelerate any deflection that occurs. Again, if you balance the bullet on the razors edge, it should in theory stay in the same spot until it's disturbed by outside forces.
Of course the center of pressure on a bullet is rarely exactly at the center of gravity. Not only that, but as a bullet accelerates, decelerates, and changes it's angles of attack and incidence (the angle between the longitudinal axis, and the direction of travel; and the relative angle between that axis, and a plane perpendicular to gravity), the center of pressure will change; sometimes radically. Changes in pressure and balance, cause instability.
Now, there is a component of the forces on a bullet called the moment of inertia. The further the action of a force is away from the center of gravity of an object, the greater its moment of inertia. A given force will have a greater effect on an object, at a greater moment of inertia.
In simplified terms, the further away from the CG the force is, the more leverage it has. The more leverage a given force has, the more change it will induce.
How does this apply to bullets in flight?
Well, the longer the bullet is, the greater the moment of the forces, therefore the larger the effect of aerodynamic forces on the bullet are; and more specifically the more they change as the rotational and linear velocity of the bullet change. Additionally, longer bullets have more chances for imperfections, and imperfections also cause changes in the effect of aerodynamic forces.
The very definition of stability is resistance to change; and the more change there is, the less stability there is.
OK, so that what stabilization is, and how it works, but why is it important?
Simply put, stable bullets are predictable.
If a bullet is stable in its flight, it is more likely to hit the same spot as the last bullet.
That is precision.
If a bullet is stable in flight, it is more likely to hit what they are aimed at.
That is accuracy.
Precision produces grouping; accuracy produces scoring (or stopping); both of which are kind of important in the application of firearms for both competitive and practical purposes.
Encapsulated: stable bullets are both more accurate and more precise. Longer bullets require more stabilizing forces to maintain stability. Faster twist means greater gyroscopic effect, and greater stabilizing aerodynamic forces.
Now, the second major myth is "Overstabilization".
Some folks believe that you can "overstabilize" a bullet, and therefore reduce accuracy. For all practical purposes, there is no such thing as overstabilization, so generally faster twist doesn’t hurt accuracy with lighter bullets...
Except in reality it does, for three reasons: unbalanced aerodynamic effects, out of balance bullets, and structural failure due to overspin.
The problem with very light bullets, its that they are more lightly constructed. This makes getting them perfect and consistent and perfectly balanced more difficult. Additionally, any imperfections there are, will have a greater effect because they do not have the mass (and thus the inertia) to resist destabilization.
The faster bullets spin, the more aerodynamic lift is generated. Though the lift is radially symmetrical, thus it balances itself out as explained above; the totality of force is still greater, and therefore there is greater potential energy in case of upset. Very slight imperfections in the bullet cause aerodynamic disturbances which upset and partially offset the stabilizing effects of faster spin.
Those same imperfections, along with slight variations in the distribution of mass throughout the bullet, also cause out of balance “wobble” or precessional destabilization (as opposed to precessional drift, which is motion in the axis and direction of rotation due to gyroscopic forces).
All of these factors upset the bullets to varying degrees, causing instability, and reducing accuracy and precision.
This is true of heavier bullets as well; but their greater inertia makes the forces required to cause upset far greater; as well as increasing the tendency to damp out upsets and return to stable orientation.
Finally, because lighter bullets are more lightly constructed, they are also not as strong; and the faster rotation of higher twist rates causes the internal stresses on the bullet to be higher (torque, centripetal and centrifugal force, shear forces between the layers of construction etc...); which may cause the bullets to disintegrate either in the air from imperfections in the bullet, or on impact; without penetrating the target.
This is common with the lightest varmint bullets driven at very high velocities, and is sometimes called "poofing", or "going poof", because when the bullet disintegrates there is sometimes a visible puff of lead and copper residue in the air. This tendency is unsurprising when you consider that a bullet traveling at 4000 feet per second, may be rotating as fast as 400,000 RPM.
Earlier I said that for all practical purposes, overstabilization doesn't exist in the real world of shooting. This isn't to say that too much gyroscopic stabilization can't occur, but that it's effects are generally so minimal as to be insignificant.
Now, some folks will tell you that you can stabilize a bullet so much, that the bullet wont follow a proper ballistic arc; the gyroscopic effect causing the bullets nose to always point upward at the original angle the bullet was fired at, and cause the bullet to keyhole the target.
To a slight degree this can be true; but only at extremely long or extremely short ranges does this become an issue.
Even some otherwise informed and reputable sources (including the ammo oracle) will tell you this is a problem but this is simply not true of almost all bullets, fired at almost all angles. You would need to have a very short, very heavy bullet, fired at an extreme angle, with a very high twist rate, and with a shape that puts the center of pressure in an odd relationship to the center of gravity, for this to significantly reduce observed accuracy at anything but the absolute shortest, or longest ranges possible.
A bullet in flight will naturally tend to stabilize in a ballistic arc, with the base of the bullet behind the nose, because as the angle of attack changes, the center of pressure will move slightly behind the center of gravity. The base of the bullet acts like the trim tab of an airplanes tail; it tends to react against deflection, and oscilate in a cycle of reducing magnitude until the various forces on the bullet balance out, and stable state is regained.
In general when looking at bullet flight, gravitational forces, thrust forces, gyroscopic forces, and aerodynamic forces will naturally find a balance of ever decreasing magnitude, to produce a smoothe arc. Gravity itself will tend to pull the nose of the bullet down, while deflection lift (equivalent flat plate effect, also called weathervaning) will tend to keep the base of the bullet up. This aerodynamic tendency should overcome any tendency to maintain the initial angle of incidence because of high gyroscopic forces. In a vacuum, that same smoothe arc exists, but the orientation of the bullet will not be stabilized aerodynamically; and the bullet will tend to remain oriented in the intial direction of firing.
The only cases in which this would not be true, is if the range were so short that the bullet had not had time to straighten out from an initial disturbance, or if the range were so long and velocity so low that combined with a very high angle of incidence, a very high angle of attack, and a very high twist rate, the aerodynamic forces on the bullet had reduced to the point where the bullets rigidity in space was stronger than the weathervane effect.
There is one particular component of force that can slightly reduce accuracy due to overstabilization; and that is through precessional drift as described above.
Precessional drift is when a rotating object tends to translate horizontally in the direction of rotation, due to gyrosopic momentum overcoming static inertia (the centrigual force of the flywheel effect pulls the flywheel sideways). If a very light bullet is spun significantly faster than required for stability, at long ranges this precessional drift can slightly reduce accuracy. The heavier, and the longer, a bullet is; the more it will resist this tendency.
There are two other common and related myths I want to address:
Boattail bullet stability, and the bearing surface myth.
Some folks believe that boattail bullets are more stable, and will stabilize at lower twist rates; for varying reasons, but often because they believe that the length of surface contacting the bore makes a difference in stability.
Unfortunately, this is entirely incorrect. Sometimes boattail bullets are more stable, especially at extreme ranges; and they are certainly more aerodynamic in general; but not all boat tail bullets are more stable in all loadings or situations, and if they are more stable, it has nothing to do with the length of the bearing surface of the bullet.
The reason why boattail bullets sometimes stabilize better, isn’t because of surface contact with the bore. Why would twist rate or stabilization have anything to do with surface contact with the bore?
The primary reason boattails can sometimes be more stable, is because the boattail base is less susceptible to aerodynamic upset; and is also less likely to be nicked by a rough crown or bore imperfection. Additionally, what imperfections there are have less effect on the bullet because they are at a smaller radius (and thus exhibit lower gyroscopic forces), and have less aerodynamic pressure applied to them
Often however, very light boat tail bullets are MORE susceptible to upset, because under some circumstances they do not exhibit as much positive stability (the tendency to damp out oscillations from a disturbance) as flat based bullets, as they have less incident flat plate area behind the center of pressure.
Hollow base bullets are even more interesting, in the effects that they have on turbulent flow, and base upset; as well as their deformation characteristics out of the bore which can cause a shuttlecock effect; but that's neither here nor there.
Some folks believe the bearing surface length is important, without regard to bullet design, but this too is incorrect.
You’ll note that the reason why twist rate increases stability, is because faster rotation means more aerodynamic forces (radially symmetric lift) as well as more gyroscopic forces. This has nothing to do with bearing area. Let me say this again: The bearing area and rotational velocity of the bullet in flight, are not interrelated.
You could have nothing more than a bearing band 1/8” long, and the bullet would leave the barrel spinning at the same rate, and with the same rotational energy (though a slightly higher linear velocity presuming gas seal integrity was maintained) as if the bullet were a perfect cylinder (presuming the rotational forces weren't so great that they distorted the bearing band of course).
In fact bearing bands are used by many large diameter (.50cal and above) rifled projectiles; as well as many projectiles used in 18th and 19th century in muzzle loaders; because the use of bearing bands reduces friction and fouling. Also some cast lead revolver bullets use a bearing band design for the same reasons.
Heck, boat tail bullets are even longer per given weight than flat base bullets are, so they would have an even faster required spin rate if it weren't for the factors I spoke of above. If you can shoot heavier boat tail bullets through a slower twist barrel accurately, it is for the reasons I mentioned above... welllll and one more thing
Now, I said up near the beginning of this piece, that form lift, or rather to be more specific aerodynamic characteristics based on shape, could be important; but it wasn't relevant to the discussion here specifically with regard to twist rate, length, weight, and stabilization.
That's not strictly true; because different shapes of bullet, even with identical weight and length, will have different aerodynamic characteristics based on their shape. It's just that for the most part, those aerodynamic differences are quite small between different spitzer bullet designs, different roundnose bullet designs, different flat base bullet designs, and different boat tail bullet designs.
Most often, bullets compared with each other will all be of a single design type i.e. they will all be pointed nose or all be round nose etc... There can be significant aerodynamic differences between round and pointed nose bullets, and flat base vs. boattailed bullets.
Some shapes exhibit greater tendencies toward positive stability. Some shapes have greater stability at lower velocities, and less stability at higher velocities or vice verse. Some shapes deal with things like shock waves, turbulence, or flow separation better than others.
This relates to boattail bullets specifically, because overall, the boattail spitzer (meaning pointed at the front with a gentle ogive, and tapered at the back with a flat base behind the taper) is the optimal form for maintaining both stability and aerodynamic efficiency in the velocity ranges we are dealing with for rifle bullets. Sharply pointed bullets with a tapered base generally deal with shockwaves and moving centers of pressure better than other bullet designs; resisting shockwave upset and turbulent flow upset better.
So yes, often boattail bullets will be more stable; but that doesn't necessarily mean they will stabilize at a lower twist rate (though sometimes it does). It really depends on the relationship between the CG and the CP of the bullet, as described above, total aerodynamic forces, shockwave characteristics etc... and it's not something that can be easily predicted by a lay person, or by anyone without a supersonic wind tunnel and measuring equipment for that matter.
Simply, other than the general differences between pointed and flat or rounded nose; and flat or tapered base bullets; those comparisons are not useful for practical purposes.
Oh wait, one more myth I forgot about?
What about tumbling bullets? I mentioned above that the myth was that the original M16 barrels and ammunition were made deliberately unstable so they would hit the target "tumbling".
This also, is very untrue. As we've described above, a bullet that is tumbling in flight is likely not to hit the target at all, and it's terminal effect will be entirely unpredictable and inconsistent.
The stabilization in air of a bullet has very little effect on it's terminal performance; except that a bullet which is unstable in air, will generally become more unstable in flesh. Bullets that are stable in air may "tumble" in flesh, or they may not; and "tumbling" itself is generally a misstatement of what happens.
Important to this discussion, rifling twist rate doesn't significantly effect the stability of the bullet once it enters flesh.
I take a much more in depth look at "Tumbling Bullets" in: Terminal Tumbling
How does this apply to actual rifles?
Well, every rifled barrel has a twist rate, and that twist rate determines how fast the bullet will spin. You choose different twist rates based on the length (which as we discussed above, is effectively the weight), of the bullets you wish to shoot.
Common MilSurp 5.56 ammunition is 55gr or 62gr, so that is what most owners will prefer to optimize their rifles for; but if you are interested in very long range shooting, or shooting larger game you want heavier bullets; and if you are interested in popping varmints, you want lighter bullets.
Of course the most common rifle chambered in 5.56, and the rifle which engendered this discussion, is the AR-15/M16.
The original AR15 specification used a 1:14 twist barrel, and 55gr bullets. It was this twist rate and loading which created the "M16 bullets tumble" myth, because the 55gr bullets were not fully stabilized with rifling that slow.
The rifling twist was shortened to 1:12 with the M16A1, then to 1:9 with the A2, and finally 1:7 with the A3 and A4; specifically to support longer bullets such as tracers (not to support the standard 62gr bullet, which will stabilize just fine at 1:9).
Of course there are other common platforms for the 5.56 round.
Early Mini-14 barrels were 1:14 (online sources disagree on some of the following dates by the way), but were changed to 1:10 in '78, then to 1:7 in '86, and to 1:9 in '95. Current production Mini-14 barrels are 1:9.
I am not aware of the current production twist rates for the Galil (the original barrels were 1:11, and I believe current production are 1:9 but I can't confirm), or SIG 550 series. The HKG36 is 1:7. The Steyr AUG was initially produced with 1:12 barrels and are currently produced with I believe 1:8. Initial production SA80 rifles also used 1:12 twist, and current production (HK revisions) use 1:7.
Bolt action rifles chambered in 5.56 or .223 are most often rifled with slower twist rates; to appeal to the varmint shooters who most often buy them. The majority are rifles in 1:14 or 1:12 rates.
Heres a link (warning, PDF), to a table with the twist rates of many common rifles.
What are the practical applications of all this?
First, you can generally safely shoot any of the common loadings (presuming the length is short enough), with most any twist barrel, except at the extreme ends where bullets will never even partially stabilize, or will disintegrate right out of the bore (which is funny to watch by the way; as your friend starts to swear "damn it, I'm SURE I had a good hold on that one and my zero is perfect" ). The loadings may not be very accurate past 25 yards, but they'll fire safely.
One should also note that completely unstable bullets will still likely hit your target at under 25 yards, and even partially stabilized bullets should be fine out to at least 50 yards, and maybe as much as 100 yards; again except at the extreme ends of the scale.
There seem to be a few critical destabilization points, or points where the natural tendency to be disturbed are greatest. These tend to be when the bullet first leaves the muzzle, and/or clears the muzzle blast, when linear velocity slows by 1/3, when the bullet drops below 2500fps, when the bullet drops below 1500fps, when the rotational velocity slows by about 1/3, and when the bullet drops under supersonic.
Since a 5.56 bullet fired from out of an 18” or longer barrel is basically supersonic until it hits the ground (presuming it's aimed at something closer than 800 yards away); and most loads will stay above 1500fps (and above 1/3 of their intial rotational velocity) out to those ranges, they aren't really something to worry about unless you are firing at extreme ranges.
Really what we're talking about is the first few microseconds, and the last few hundreths of a second of travel, when the bullet is leaving the muzzle or muzzle blast, when it drops below 2500fps, and when it loses 1/3 of its initial velocity (which are often very close to each other in the AR platform)
Generally speaking, the ranges that we're concerned about as destabilization points, are 25 yards, 50 yards, 100 yards, 200 yards, 300 yards, 600 yards, and 800 yards.
For example, a 75gr bullet fired out of a 1:12 barrel will be severely understabilized from the moment it leaves the bore, and will tend to go at least partially unstable in under 25 yards (pretty much within microseconds of leaving the muzzle); and may become completely unstable, not even hitting your target; though 25 yards is so close it is likely the bullet will at least impact a man sized target.
A 62 gr bullet from the same barrel will be partially stabilized, and will most likely maintain stability out past 50 yards, and possibly out to 100 yards (though not much if any further, in fact possibly not quite reaching 100 yards).
Why can't I say it will absolutely not be stable? Because every bullet is different, and because instability is unpredictable (that is after all, its literal nature). You can only talk about what is likely.
The same 75gr bullet fired from a 1:9 barrel will most likely not go unstable 'til well past 100 yards, and even may be stable out past 300 yards (depending on bullet design).
From a 1:8 barrel that bullet will pretty much remain stable until it hits the ground or drops below supersonic (more than 800 yards out typically). The 62gr bullet will stabilize completely from that 1:9 barrel, and not destabilize til past 800 yards as well.
Speaking in gross generalities; we can talk about what twist rates are likely to properly stabilize what weights of bullet; given conventional bullet design and construction.
The 40gr and under varmint bullets will fully stabilize with 1:14 or sometimes even 1:16 barrels; and will tend to overspin and disintegrate with anything faster than 1:12 or so. 45-50gr bullets will generally fully stabilize with 1:14 twist, but may require 1:12, and will not tend to overspin until 1:9.
55gr milsurp bullets will partially stabilize (out to 100 yards or so) at 1:14 and fully stabilize out past 400 yards at 1:12 (for shooting at longer ranges, a faster twist is recommended); but do not generally overspin until 1:7 or thereabouts. I haven’t seen a 55gr milsurp disintegrate from overspin in a 1:7 barrel, but there is a small but clear reduction in accuracy at long range between a 1:9 barrel and a 1:7 barrel with the 55gr bullets.
62gr bullets will partially stabilize with 1:12 to the point where they will group at 25-50 yards, and generally at least hit the target at 100 yards (though the groups will be more like shotgun patterns), but really need 1:10 to fully stabilize. They will not overspin with any barrel I’ve seen.
Most 68-69gr bullets will do all right with 1:9 but do better at long range with 1:8; though some will stabilize at 1:10.
Most 70-72gr will also stabilize at 1:9, but do better with 1:8.
Some 72-75gr bullets will do well with 1:9, some need 1:8.
Bullets heavier than 75gr really need 1:8 minimum and prefer1:7.
77-82gr bullets generally need a minimum of 1:7 (though the 77gr SMK will at least partially stabilize at 1:9 and do reasonably well with 1:8) ; and I’ve even seen 1:6 barrels. I know there are 1:4 barrels, but I've never seen one.
Generally speaking bullets heavier than 72gr are close to critical OAL for proper feeding from an AR. Most 75gr bullets are ok, but most bullets over 75gr will not properly feed. Some bullets as light as 72gr won't feed, and some 77gr bullets will (again, the 77gr SMK); depending on construction and profile. I don't know of any bullet heavier than 77gr that will proper;y, reliably, and consistently feed from a full AR magazine, but there are 82gr loadings that will single feed in match rifles.
There are bullets available for .223 at up to 102gr weight; but those are for long throat single shot match rifles; and will not magazine feed from any rifle I know of. I would presume it is these bullets that the 1:4 barrels are made for.
Now, there is a rule of thumb formula (the Greenhill formula) , which can be used to estimate the spin required to stabilize a given bullet:
150 x diameter squared, divided by bullet length = required spin
So for a .224 caliber bullet .6 inches long:
150 x .224^2 divided by .6 = 1 in 12.5 inches
The formula can be reversed to derive maximum bullet length which can be stabilized by a given barrel twist.
The formula changes to:
150 x diameter squared, divided by twist rate
Example for a .224 caliber barrel of 1:12 twist:
150 x .224^2 divided by 12 = .627
The barrel will stabilize a bullet .627 inches long, or shorter (which happens to be about the length of a 55gr bullet).
Ok, so what should I buy, and what ammo should I shoot through it?
Weeelll, that's a toughie. Whatever platform of rifle you choose, you should optimize it for the general use case. Will you most often be plinking with milsurp ammo, or are you going to be busting chucks all day long with painstakingly hand assembled match grade varmint rounds?
The first consideration is surplus and bulk ammunition. The most common surplus and bulk loadings use 55gr and 62gr bullets. 55gr loadings overspin a bit at anything faster than 1:7, and slightly overspin at 1:7. 62gr ammunition won't properly stabilize with twist rates slower than 1:9 (actually 1:10, but that's not a commonly available twist), and works fine with most any faster rate. This would suggest that if you want to shoot MilSurp, a barrel of 1:9 or 1:8 would be your best choice.
Why not go faster?
As a civilian, you won't likely be firing armor piercing or tracer rounds. Also unless you are going to be firing at very long ranges, or at game larger than coyotes, you don't need to use the extremely long and heavy bullets which need the faster twist rates.
Based on those two assumptions, you don't need to have a 1:7 twist barrel; and if you DO have such a fast barrel, you can't really shoot anything lighter than 55gr bullets out of it(and really should stick to 62gr or heavier).
Of course if you ARE shooting at game bigger than Coyotes (men are significantly larger than coyotes), or at ranges longer than 300-400 yards (and especially longer than 600 yards); then you are probably going to want to shoot longer heavier bullets.
The single most effective defensive loading available for the AR platform rifle today is the 77gr BTHP Sierra Match King, or 77gr Nosler special purpose BTHP (sold as the "custom competition") loaded to about 2850fps. This is the spec for the Mk262 loading, developed for the JSOC special purpose rifle; and is effective at both short and long ranges, from both short and long barrels.
The 77gr bullet design used in this loading, is very specifically the longest design of it's profile which will feed properly and consistently in the AR platform(many 77gr bullets will not). It has the most consistently effective terminal performance from all barrel lengths of any 5.56 ammunition.
The only problem? The 77gr bullet used, requires 1:8 or 1:7 to stabilize properly. It will partially stabilize out to 100 yards with 1:9 and 300 yards with 1:8, but it really needs a 1:7 barrel to fully stabilize past 300 yards (of course most folks won't be shooting past 300 anyway...)
So if you plan on shooting anything from 55gr to 77gr bullets out of an AR, a 1:7 is just fine; you just can't really shoot anything lighter. If you only plan on shooting 55gr and 62gr... or maybe out to 68gr, then go ahead and get the 1:9, and you'll be able to shoot some of the heavier varmint loads as well.
If you are going to be primarily varminting with the rifle, then I recommend you choose a 1:12 twist barrel, unless you are planning on primarily using the very lightest 37-42gr ammunition, in which case you should go with 1:14. The 1:12 is a versatile twist rate that will allow you to fire even the lightest varmint loads, while still preserving the capability to use 55gr surplus, and in an emergency even 62gr ammunition.
If you're going to do a little bit of everything, buy a 1:9 barrel. You can shoot everything down to the 45gr varmint loads, and up to the 68gr hunting lods, and still get good results.
So, any questions?
Oh, and in case you were wondering, this post clocked out at about 4900 words. Only half as long as my longest, but still a stiff hit. It took me about 3 continuous hours to write it, then another two hours or so of re-writes.