I don’t consider myself more than a hobbyist when it comes to ammunition – I reload a little, play around in SolidWorks a bit, and read dry, dusty tomes full of other people’s hard work collating every minute detail about ammunition. I’m, frankly, an ammo nerd, but not really a true expert. Once I began writing, however, I found that very few people are ammunition nerds, at least in the same way that I am.

As an aside, this is neatly illustrated when you compare IAA Forum’s membership – about 3,200 at the time of this writing – with AR15.com’s, which is coming up on half a million. Keep in mind, too, that while IAA Forum is probably by far the biggest ammunition collecting/information forum on the Internet – at least so far as I know, AR15.com is just one out of many giant gun forums on the web.

Anyway, over the course of being involved in the gun world as both a shooter and writer, I’ve heard a lot of myths about ammunition and ballistics,  some of which are obvious to most people, and some of which get repeated a lot more often than they should. What follows are some of these myths, and the truth behind them.

1. Bigger is Better

I’ve put this one first because it is easily one of the most common. This is a myth that won’t die because it’s so viscerally apparent. If you have some handy, take a .45 ACP and compare it to a 9mm, or a .308 Winchester against a .223; any two cartridges that are significantly different in size and weight will do. It’s so obvious, in a way that is maybe hard to explain, that the bigger round is better, that it will do so much more damage. A .45 ACP hardball in your hand, all three-quarters of an ounce of it, just feels so much more substantial and powerful than a 9mm, or a .32, or another smaller round.

I won’t spend much time speculating as to why – maybe it goes all the way back to our ancestors plucking stones from the river to throw at game birds – but I think this reaction is what keeps the idea alive.

Regardless of the reason, the terminal ballistics of different projectiles is a complex subject, and often things shake out differently than just the size of different rounds would suggest. High velocity rifle projectiles that fragment violently as they enter the target, for example, may produce far more catastrophic wounds than much larger and heavier large-bore calibers, especially against thin-skinned targets. Expanding jacketed hollowpoint rounds even in smaller calibers like .32 may well crush and cause more tissue destruction than even a .45 cal hardball. Even the contour of a projectile can change its effect on tissue, as a flat, angular projectile may cut and crush tissue better than a round-nosed projectile in a larger caliber.

None of this is to say that bigger calibers are never more effective, or that all things being equal and with modern fragmenting or expanding bullets they will not be more effective to a certain degree, but instead the take away should be that terminal ballistics is a subject with considerable depth and complexity, and that often the actual performance of different rounds will defy expectation.

2. Longer Barrel = Proportionally Higher Velocity

This is one that is as intuitive as it is wrong. If you double the length of a gun barrel, you double the velocity, right? It will probably seem obvious to my readers that this is not correct, but there are many people who repeat it (in fact, gun designer Loren C. Cook repeated this same myth when promoting his submachine gun). It’s an obvious extrapolation from the information that longer gun barrels (often) produce higher velocities, though erroneous.

The relationship between barrel length and muzzle velocity is actually very nuanced, but it can basically be summarized as this: When the cartridge’s powder ignites, it creates gas that expands against the base of the bullet. When the bullet is seated in the case against ignited powder, the pressure is high, but as the pressure pushes the bullet down the barrel it spends its energy and also the volume of the firing chamber increases steadily. That means that the change in muzzle energy per inch of barrel starts very high with shorter barrels and steadily decreases as the barrel gets longer. Going from an 10″ to a 13″ rifle barrel can mean an increase in velocity of hundreds of feet per second, for example, while going from a 21″ to a 24″ barrel can mean an increase in velocity of just a couple dozen feet per second. You’ll commonly hear the change in pressure and force on the base of the bullet referred to as the “pressure curve”.

Further, this curve and its relationship to the length of the barrel varies for different propellants. Magnum rifle calibers tend to use very slow-burning propellant, which cause significant changes in velocity even when going from a long rifle barrel to an even longer one. Pistol calibers, in contrast, use extremely fast-burning propellant, meaning that after only a few inches the increase in velocity as the barrel gets longer becomes negligible. In fact, firing a pistol caliber from a longer rifle barrel can actually decrease the muzzle velocity slightly versus a shorter barrel, as the friction between the bullet and the bore begins to retard the bullet’s travel more than the additional pressure accelerates it.

3. Caliber Matters, Bullets Don’t

This is an odd conceit that comes up very consistently in conversation, usually expressed in the form of “[caliber X] isn’t enough. You need [caliber Y]”, where the two calibers in question are pretty similar to one another. While it’s certainly possible for one to choose a caliber that is truly inappropriate for the job in question, it seems that the majority of the time these discussions revolve around calibers that are all more or less appropriate, given proper bullet selection.

That’s where this goes beyond fussiness to being an actual myth: These discussions would virtually all be more productive if they accounted more for bullet selection than caliber and chambering. There is, after all, far more difference in effectiveness between a .45 ACP hardball and a .45 ACP HST, than there is between a 9mm HST and a .45 ACP HST. The choice of one chambering or another is not likely to make a tremendous amount of difference to the end result, but the choice of one projectile over another is!

4. Momentum = Stopping Power

Momentum, that is the product of mass times velocity, is a very easy-to-grasp physical quantity. A bigger man bumping into you on the street at the same walking speed will push you aside more violently than a smaller man. A larger rock or stone thrown with the same velocity will make a bigger splash in water. It’s a simple quantity that’s easily calculated and readily understood. The bigger something is, and the faster moving it is, the more oomph it has.

That’s why it’s very natural for momentum to be used as a rough metric for stopping power. This connection is made all across the gun world, from gun reviews noting that a bigger round rings steel more loudly than a smaller one, to the Taylor Knock-Out Index which combines momentum with the diameter of the bullet in an attempt to quantify stopping power against big game. However, although momentum is an important quantity in ballistics, it does not imply the degree of terminal effectiveness, or “stopping power”, that a projectile has.

Momentum is a conserved quantity, which means that as a bullet is driven forward by the force of expanding gases, the gun firing that bullet is also driven rearward with the same momentum as both the bullet and propellant gases combined. This means that the momentum produced by any bullet fired from a shoulder or hand fired weapon is not enough to significantly wound a human being, much less kill. The momentum of a bullet, when it strikes a target, does not do more than perhaps somewhat bruise the surrounding tissues and very slightly accelerate the target rearward. The killing power of a gunshot comes instead from the velocity at which the bullet is driven, and the permanent cavity created by the projectile’s material carving a wound channel into the target.

This topic is deliberately attention-getting and broad, as I’m considering covering more topics in this vein on all levels from introductory to advanced, and I am looking to see what kind of response I will get for subjects like this. If you’d like for me to cover more ammunition- and ballistics-related topics beyond this one, please let me know in the comments section.