That's a pretty interesting question (that I've partially addressed before). Particularly, it's a complicated and kinda hard to answer one. For one thing, there are a lot of performance tradeoffs; and that question I've already talked about elsewhere, and will again; but I'm not going to take the time to talk about it here except in the most basic way.
What I do want to talk about, is how do you know how a particular load will shoot in your gun?
The short answer is, you don't until you try it.
The most important criteria for any carry ammunition is reliability. You have to be 100% sure that when you squeeze that trigger, the weapon will fire, and it will cycle and fire the next round, and the next after that. The loudest sound in this world is a click where there should have been a bang.
The second most important criteria is precision. No, not accuracy; accuracy is the tendency for a bullet to impact close to it's point of aim, and it's important; but REPEATABILITY is more important. Repeatability is also called precision; and it's the tendency for multiple bullets to impact the same spot time after time (which makes groupings). Point of aim can be adjusted to produce the desired result, but if a given load could impact anywhere in an 8" circle of where you are aiming at 10 yards, then there's a lot of ways for you to miss.
The third criteria is controllability. If you can't control the weapon, with reliability and precision; at all angles and speeds of fire; then that load is not suitable for you, with your weapon.
Finally, the last criteria is terminal performance; and really that's just a guess. There are multiple wounding theories out there, and very little scientific understanding of the details of terminal performance; but you can speak in certain generalities:
- Modern premium jacketed hollowpoint ammunition is generally better than FMJ
- Within a given caliber, faster is better
- Within a given caliber, heavier is better
I could tell you which is more important; faster, or heavier; but no-one is really sure, because in different circumstances and with different bullets, either one could produce a better result (I've written a lot more about this elsewhere, as have hundreds of others). Honestly, I wouldn't worry too much about it. I choose the heaviest +p or +p+ load that I shoot well, and that each gun likes; and I'm confident that it will produce a good result.
So back to that question, how do you know what load is going to be reliable, and shoot well in your gun?
Every gun is different, even the same identical make and model weapon, and will shoot differently with different ammunition, and of course every shooter is different; so the data I collect for my HK USP, may not be useful for your particular HK USP, and almost certainly wont be useful for your 1911.
Well, that's not quite true. If velocities are consistent with my gun, they are likely to be consistent with your gun, presuming the guns are the same (or very similar) types; because consistent velocities are an indicator of well assembled ammunition with good quality control. Conversely inconsistent velocities are often an indicator of poor ammunition; unless the inconsistency spreads across multiple loads (in which case you should see a gunsmith).
So, what does that mean? Yup, you guessed it, it's testing time.
This is the fun part... expensive, but fun.
When I get a new hand gun (the process for rifles is similar, but more anal, and thankfully involves fewer rounds), the first thing I do with it, is shoot 200-500 rounds of generic practice ammo or practice handloads through it; to break it in, and to get a reliability and functionality baseline.
In that first batch, I will typically follow a basic break-in procedure for the first 100 rounds (5-20 rounds, then a quick clean with a bristle brush and bore snake, to get any filings that come off with the break in, and keep the initial powder and copper fouling loose; then another string etc...); then I’ll shoot the rest without cleaning at all, to test reliability when dirty. I will test the weapon in adverse positions, in rapid fire, etc… to get a feel for the weapon, and again to help understand the basic reliability.
Be careful what you read into that reliability or accuracy data though, because the weapon may be plenty reliable with premium defensive loads, but unreliable with the white box loads (for any number of reasons).
For example, Winchester white box .45acp is typically loaded about 10% below SAAMI pressure standards for the round (not the top pressures, the standard pressure). I have two auto pistols that are built for .45 super, and they will sometimes have failures to cycle with that ammo, because the low pressure loads aren’t strong enough to be reliable with the high pressure springs in those guns. Remington green box hollowpoint rounds don't like to feed in those guns either, but in other guns I have work just fine. Magtech, Wolf, and Independence ammo both tend to be loaded to very low pressures (even below WWB), and also tend to have inconsistent velocities; which makes them both unreliable, AND imprecise, in a lot of guns (including mine).
On the accuracy side, most of my guns print exactly to point of aim with the +p and +p+ loads I carry them with; but they shoot low, and often a bit off to one side with the generic white box ammunition (no, I’m not sure of the explanation for that. It should be a bit low, but in the same vertical line, unless the pressures and velocities are WAY low. If they don’t shoot to POA out of the box with the load I’ve chosen, then I’ll adjust or reshape the sights -in the event of a sight that isn’t at least drift adjustable- so that they do; but that's later).
This is one of the reasons it’s so important to practice regularly with your carry ammunition; because under stress you will revert to training, and you need to train to shoot straight, not to hold Kentucky windage; and especially not to hold two different points of aim for different loads. It's also the reason why you want to thoroughly qualify a couple of loads (preferably those that group close to the same point of aim), then stick with them.
Once I’ve completed that process, I shoot for groups with the ammo I intend to carry the gun with (or shoot in matches if it’s a competition gun; but for IDPA I only use carry ammo; or reloads to the same spec for practice purposes. I don’t currently shoot USPSA, which is typically shot with specialty competition loads).
In particular, I will shoot for group with number of loads (at least five loads from several different manufacturers if possible) shooting at least four 5 shot groups per load (conveniently, one full box of premium hollowpoints) off a fully supported rest; to test how the gun functions with that ammunition, and to obtain useful accuracy information.
If possible, I like to shoot through a chronograph for this testing; so I can get an idea of the consistency of velocity (mean, median, standard deviation, and extreme spread). It's not particularly important by itself in a defensive handgun so long as the load groups well, but it's good to know.
Once I’ve narrowed the load possibilities down to two or three (they almost always end up being CCI/Speer, Cor-Bon, Hornady, or Federal premium/Hydra-shok), I will run at least 100 rounds of each load through; checking for function at all angles and positions, limp wristing, rapid fire, with my thumb riding on the slide etc… getting an idea of functional and feed reliability for that load.
Importantly, I’ll shoot each of these loads through all of my carry magazines for the gun (and all the practice mags if I can); and check for feed and cycle reliability.
I will also shoot controlled pairs, and controlled triples (mozambiques), as well as other drills, to get an idea of rapid fire controllability and accuracy.
If the weapon has more than one malfunction that I didn’t cause deliberately, or if it malfunctions too easily etc… then that load isn’t the right one for that gun. If the accuracy, precision, reliability, and controlability prove acceptable, then that load has been qualified.
I like to qualify at least two loads for every weapon, and I prefer three if possible, so that I am more likely to be able to find a qualified load wherever I go if I have to. I also like to make sure at least one of those loads is one I can get pretty much everywhere locally, and even in Wal-Mart if possible (most Wal-Marts don't carry much of a selection of premium hollowpoints, if any at all).
At that point, I’ll adjust the sights (permanently or otherwise) to shoot to POA with my chosen primary carry load; and then we’re done.
Yes, this is an expensive process. It usually costs me approximately $250 in ammunition to qualify a load with a gun; and that data is not useful to that load with another gun (though it is at least instructive). I've had guns where I had to spend over $500 in ammo jsut to find a laod that the weapon liked.
The most expensive bit was qualifying my guns with the Glaser safety slugs. I was only able to do so because I bought about 1200 of them (in various calibers) at auction for $200; and in fact I've only qualified four guns with them.
I don't recommend that anyone shoot Glasers through an auto pistol, unless they can test with at least 50 of them (and preferably 100); and at the current pricing that would cost somewhere around $200. Even with revolvers (where you don't have to be so concerned about function) I'd still recommend testing with at least 20 rounds, and probably more, because Glasers shoot to a RADICALLY different point of aim than conventional ammunition (they are far lighter, and loaded to a far higher velocity).
So yeah, it costs... but if I’m going to carry a weapon to defend my life, and the lives of others, I need to be ABSOLUTELY certain that the ammunition I choose is both reliable and precise.
That certainty is cheap at any price.