Vantage Point Fizzics ‘R Us
A shipment of Liberty Civil Defense “light and fast” ammo
got Tom thinking about the whole speed versus heft thing.
I’m not a physicist, but I did read Astrophysics for People in a Hurry by Neil deGrasse Tyson. He attempts to explain such things as how, at the moment before the Big Bang, all of the matter and energy in the known universe was contained in a space a trillionth the size of the period that ends this sentence. Now that I think about it, trying to wrap my head around stuff like that is likely why I’m not a physicist. I can’t even comprehend a miniature Reese’s Peanut Butter Cup being packed into a wrapper that small, much less the sum total of all stuff that ever existed anywhere in the cosmos.
While the deGrasse Tysons and Hawkings of the world can ponder the great expanse in their studies, we mere mortals can make more practical use of the physical sciences for our shooting activities.
Smokin’ Fast
The folks at Liberty Ammunition recently sent me some Civil Defense ammo. As a quick refresher, the ballistic theory behind this particular ammo style is “light and fast.”
The 9mm uses a 50-grain projectile, so you’d expect it to carry more velocity than the more traditional projectiles weighing over twice as much. Fired from a 9mm 1911, I measured this one at 2,063 fps — significantly faster than Maverick’s F/A-18 Hornet (1,745 fps).
The .357 SIG version also uses a 50-grain projectile, and given the larger case size, you’d expect it to make the 9mm look slow. It does. I clocked this one from a SIG P320 at 2,404 fps.
I also tried some of the “heavy” stuff. The 10mm offering uses a really chunky projectile, weighing in at a whopping 60 grains. From a Springfield 1911, it measured 2,377 fps.
Let’s stop and think about that for a hot second. We’re talking low-end rifle speeds. Or if you’re a Top Gun fanatic, over Mach 2. If I hadn’t just read an astrophysics for dummies book, I might have asked a stupid question like, “Are there two sonic booms when you shoot it?”
Light, Fast, Heavy, Slow?
All of this got me thinking about the never-ending debate of which is better — heavy and slow bullets like a .45, or something lighter but much faster, like maybe a Civil Defense round. Yeah, we all know a stray .45 ACP round destroyed the city of Dresden in WWII, but that’s just one anecdote — we need more information to settle the debate.
The 9mm Civil Defense round (left) weighs just 50 grains but moves along at
2,063 fps compared to a 230-grain .45ACP, which flies at a leisurely 850 or so.
No Favors
Pondering this and how many .45 bullets can be squished into the size of a period, it occurred to me that we don’t help ourselves with how we rate ammunition.
The traditional standard is “energy,” meaning kinetic energy. Lots of 9mm rounds have “energy” of 340 or so to 400 foot-pounds for the hot loads. A traditional 230-grain .45 ACP moving at 850 fps generates 369 ft-lbs. You get the idea. But what does that tell us? I put forth … not the whole picture.
Energy Vs. Momentum
Let’s make an oversimplified and likely defective analogy. All you physicists out there, no need to send the correction emails; we’re just dealing in gross illustrations.
One might think of kinetic energy as destructive power, like an electric drill. It’s moving fast and will wreak havoc on drywall, a 2×4 or a bowl of Jello. But on its own, it will not knock down said objects or fling them into oblivion like movie characters hit by handgun rounds. Except maybe the Jello.
For momentum, I’d use a rough description of “the ability to move an object.” If you roll a golf ball at a bowling pin, the effect will be far less drastic than hurling a 16-pounder at the same velocity because the momentum is dramatically different.
Energy
If you look at the math behind kinetic energy, measured in ft-lbs., it emphasizes velocity. You calculate it as one-half times mass times velocity squared. This is precisely why a 9mm easily matches the “energy” of a .45 ACP bullet twice as heavy. It also explains why a .223 Remington cartridge, with its puny little 55-grain bullet, generates almost three times as much energy (1,100 ft-lbs.) as a 9mm or .45 ACP.
Momentum
On the other hand, momentum weighs mass and velocity equally — the equation is — mass times velocity. The weight of the bullet counts a lot more in the numerical result.
For example, a 9mm 124-grain bullet moving at 1,150 fps has about the same energy as a .45 ACP traveling 850 fps (364.2 vs. 369 ft-lbs.), but the .45 has roughly a third more momentum (27.93 vs. 20.37 lb.-ft/sec). There — a victory for the big and slow ballistics crowd!
But … momentum doesn’t tell the whole picture either. The momentum of a 115-grain 6.8 SPC round traveling at 2,675 fps (43.95 lb.-ft/sec) is almost exactly the same as nine bananas dropped from six feet (44.21 lb.-ft/sec). I’ll elect to stand in front of the bananas, given the choice.
The Answer?
I suppose we could launch an extensive campaign to encourage ammo manufacturers to print both measurements on the box. That might prevent confusing implications resulting from our traditional ammo power measurement. As we all know, a pro bowler wings a 16-lb. ball at almost 18 miles per hour. That generates approximately the same kinetic energy as a .380 ACP, and a single .380 ain’t knockin’ down all 10 pins. If it were clear the bowling ball delivered 422.4 lb.-feet /sec of momentum, compared to just 12.21 for the .380, things would make a lot more sense.
