Wednesday, December 03, 2008

Sectional Density And Bullet Penetration.

The other day I made an offhand comment on another blog, Pamibe's if it matters, about using heavy for caliber bullets. This, unbeknown by me, started a bit of an argument over what I was talking about. I keep forgetting that most people, even most shooters, aren't ballisticians.

Ballistics has nothing to do with the scene on cop shows where they find a gun or a bullet and say "take it to ballistics", that is forensics. Ballistics is the science of the moving bullet, divided up it to three parts, interior, exterior and terminal. Interior ballistics is the science of what happens between the time the primer pops until the bullet exits the muzzle of the barrel. This is all acceleration and pressure. We care about that because if we have too much pressure the shootin' iron might blow up or the bullet might go too slow to do anything useful. Or, for that matter, go so fast it blows up in midair.

Terminal ballistics is what happens when the bullet hits the target. In many ways this is the hardest part of ballistics because there are so many variables. Impact velocity, bullet weight, bullet shape, bullet material, how thick the bullet jacket, hardness of the bullet core, how stable the bullet is, the media that the bullet hits, etc, etc, etc.

One of the most important things to know in ballistics the the sectional density of the bullet. This number is a simple equation, the bullet weight in pounds divided by the square of it's diameter in inches. So, a 180 grain .30 caliber bullet is .0.0257143 of a pound. So we divide that by .008992 and get .270. Actually we just look in the loading manual where this has already been done.

In the days before the fancy new bullets like the Swift A-Frame and the Barnes Triple Shock or even John Nosler's Partition, the first of those new ones, first made in 1949, the only way to increase bullet penetration was to increase bullet weight in that same caliber. Roughly speaking any two bullets of the same shape, construction and velocity, with similar sectional densities will penetrate the same distance.

For instance a .270 Winchester 130 grain bullet has roughly the same sectional density as a 165 grain .30 caliber bullet. The .270 is .242, the .30 is .248. This is why that famed gunwriter Jack O'Conner loved the .270. If the same critter were shot with both bullets they would each penetrate the same distance, only the .270 wouldn't kick as much.

Until hunters forgot how to stalk game the class of the hunting fields used moderate velocity, heavy-for-caliber bullets.WDM "Karamojo' Bell, the famous old ivory hunter slew bazillions of elephant with the old 7 mm Mauser and 6.5 mm MS. The long solid bullets penetrated just as well as the elephump guns. They left the guns at the same velocity as those big bores, about 22-2400 feet per second. Bell kept a big gun handy in case he had to stop a big critter but mostly he snuck up and popped a bull in the brain. This, of course, before the idiot environmentalists got in the game. Now, instead of Jumbo being a valuable resource he is a pest. Where before a Jumbo would add thousands in trophy fees, plus each ne feeding entire villages, they are poached, with no trophy fees for the governments, most of the meat rotting. Oh well, it makes liberals feel good. But I digress.

I shall look through the Speer loading manual at bullets with an SD of about .240-.250, each will penetrate similarly. The 100 grain 6mm bullet, the 155 .257 bullet, the 120 grain 6.5 or .264 bullet, by the way, those fine old 160s will still hit today and pentrate 'til a week from Tuesday, the .270 130 grain mentioned above. A 140 grain 7mm bullet, but if you want to drive really deep, the 175 grain works well.

Another aside, if I were wanting a hunting rifle that would take anything on this continent (except for Grizzly and Brown Bear in heavy cover where they might bite) and didn't want to be kicked silly by recoil I'd want a Seven mm Mauser or .280 Winchester and load the Hornady 175 grain round nose soft point to around 2400 fps and limit my shots to 200-250 yards.

Anyhow, the list goes on, the fine 165 grain .30 caliber, the 170 grain of the old .32 Winchester Special, the 200 grain .338 for those that want to use a moose and bear gun for deer, a 220 grain .35, the 235 grain .375, and the 350 grain .458. If we assume that all else is equal, each of these bullets will drive about the same distance through similar mediums, whether that be flesh, bone, old newspapers, ballistic gelatin, etc. A number that sticks in my head from my teens is that the original WW1 service load for the 1903 Springfield is that those old .30 caliber solids would penetrate sixty-one inches of pine board. Kind of plays the fool of the old cowboy or war movies with folks hiding behind a wooden wagon or empty barrel. Five feet of pine boards, space an inch or so apart. That was a 172 grain solid back in the day when the fastest such a bullet could go was maybe 2650 fps.

Handgun bullets, being shorter than rifle bullets of the same caliber, have lower SDs. My two favorites the .357 and the .45s have SDs ending at about .180. for the standard heavy bullets. I load a six shot .357 revolver with the 125 grain Federal hollowpoints when I'm loading for serious business. My little five shot Ruger SP101 gets a handload, 5.1 grains of Bullseye for an average of 1007 fps with the Speer or Hornady swaged lead semiwadcutter hollowpoint. Note, I live where my DA only cares about why you shoot someone, I've read of jurisdictions where it is financial suicide to shoot a bad guy with a handload.

For my .357 Carbine I like the Hornady 158 grain XTP hollowpoint. I've never had one stay inside our smallish Texas Whitetails, no matter the angle. I keep thinking about trying the 180 grain Remington hollowpoints but then I get distracted and forget to buy some on the rare occasions I have any money. The SD of the 125 grain .357 is .140. At full velocity a good hollowpoint will seldom exit a grown man. The soft point will. The old Remington medium velocity 125 grain hollowpoint, at about 1000 fps from a four inch revolver actually penetrated a little further than the full steam loads at 1450 fps. The bullets expanded more with the hot loads so, pushing a larger frontal area they lose steam a lot quicker.

In my .45 Colts I use the same bullet weight in all my guns, 250-255 grain. The difference is, in my handguns I shoot softcast or swaged lead bullets at standard velocities, about 8-900 fps. My rifle shoots those loads well, for play. At a cowboy action shoot those loads ring the steel of the targets just fine. A slight sight adjustment bring up the hot loads, though. A max load of Hodgdon's Lil Gun or Hodgdon's H110 will fling a 250 grain Hornady XTP hollowpoint out the muzzle at close to 2,000 fps. Now that is very close to the old Trapdoor Springfield Cavalry Carbine Ballistics, lower velocity with the Springfield of course, with a heavier bullet.

These days bullet weight is not as important as it used to be, todays fancy bullets penetrate as deeply as the old heavy bullets. John Nosler invented his Partition bullet after the failure of an old cup and core bullet failled in his .300 H&H Magnum. Today a 130 grain Barnes .30 will penetrate like a 165 grain cup and core jacketed lead bullet. The thing is, those fancy bullets not only cost a lot but they are very much attuned to impact velocity. You can't hardly drive a Barnes fast enough to hurt anything but if the velocity is too low they just pencil right through. With the old bullets the bullet weight is always there. If the critter is too big for a 150 grain .30, those critter will fall with a 220 grain soft point.

 Update: I just noticed that comment about Terminal vs. exterior ballistics. Semper fi guy is correct, I don't know how I made that silly mistake, except that I usually write these posts in a house full of dogs and a wife, distracting me. I shall leave that comment, he's not quite correct though. I ain't all that nice.


thedoveshooter said...

Amen. This is why I shoot a 200 gr. Speer Hot-Cor in my surplus 8x57 Mauser. It shot clean through two hogs' shoulders. Can't as for much more.

RalphMal said...

But wait, what about momentum?? Take two different caliber bullets with same sectional densities and same velocities, and all other things being equal, the heavier (larger caliber bullet) will penetrate further because of higher momentum.

Anonymous said...

Shakey Pete

is blowing smoke.

As soon as he made this comment (early in his post), "Exterior ballistics is what happens when the bullet hits the target," my eyes glazed over. He has exterior ballistics confused with terminal ballistics.

He's a Nice Old Guy, but not to be taken seriously. At least, not about sectional density, ballistics, and such similar items.