Sarah Palin knows how old the Chinese gymnasts are.
I have no idea why this was on CNN’s news page today. It’s a link to a story at Cracked from 10 months ago about the false ideas Hollywood spreads about firearms. As if you didn’t already know. Just for gits and shiggles I guess.
The bit about silencers is good, but it could use a lot more detail. They do mostly tell you the truth: there is no such thing as a silencer. In other parts of the world where such things are legal, even required for hunting in many countries, the devices are called suppressors. Because that’s all they do. They abate a noise loud enough to instantly damage your hearing down to a noise only loud enough to damage your hearing if you’re exposed to it for an extended length of time, like a whole 2 seconds. Guns are loud; wear double ear protection when you go shooting.
There is more to the gun loudness issue than just the boom that the gunpowder makes. Any bullet that travels faster than 1100 feet per second - and most do - is flying faster than the speed of sound, and that means it creates a sonic boom. Even though bullets themselves are relatively small, the noise of their passage is significant. This is why real “silenced” guns shoot slow bullets, around 950fps. And the only way to get any kind of power with a slow bullet is to use a really heavy bullet. Unfortunately heavy bullets at really low velocities aren’t usually fully stabilized, so accuracy suffers. Bottom line is that a truly silent firearm isn’t going to be accurate enough or powerful enough to get the job done at any kind of realistic range. 75 yards, maybe 100, and that’s really pushing the envelope.
Why don’t silencers work? It’s a simple matter of volume. Hur hur hur Drew, good one. No, seriously, it is. Not the “turn down the volume” kind, the “cubic feet of air” kind. To be effective they have to contain all the gas that comes out of the end of the gun and then release it to the atmosphere slowly enough so that there is not pressure wave. That’s the bang sound; it’s the air rushing back in to fill the volume displaced by the expanding powder gases as they leave the end of the barrel. It’s a small thunderclap.
Here’s the math in a simple example. (Sorry Rich, sometimes math is necessary)
Ok, to actually silence a firearm you have to capture all the gas that comes out the end of the gun. Other than using a subsonic bullet, that’s all there is to it.
Typical groove diameter for a 9mm pistol barrel is .355”. With a 5” barrel this means that the volume of the barrel is 3.14159 * (.355/2)2 * 5 = 0.49489 in3. Call it half a cubic inch.
To capture that much gas, you need a vacuum box attached to the end of the barrel. Since no vacuum is perfect, you want to design it a little oversize ... so you’d need an airtight box of about a cubic foot to do the job. You want to handle the heat as well; gunpowder burns at a temperature higher than that needed to melt steel. Good old PV=NRT takes care of most of that; as the gas expands it cools off. But build the vacuum chamber a bit bigger than math requires just to be on the side of certainty. Naturally you’d stick in all those nifty internal baffles to deflect the ejecta blast and to stifle the muzzle flash. And you’d need to figure out the right kind of membrane for both ends that the bullet could pierce without impacting accuracy too much. Maybe Mylar film would work. And you’d need an evacuation valve so you could pump out the air and hold the vacuum. And all of this would be good for exactly ... one shot.
Not what I’d call a practical size. And this is for a puny little pistol. A hunting rifle runs at double that pressure so it would require a much bigger chamber; something about the size of a garbage can ought to do it.
So follow the math and follow the link, and take home today’s lesson: Hollywood feeds you lies about guns. About everything else too, but that’s another lifetime’s worth of posts.
Ok, that’s enough school for one day.
I realized that I “cheated” because I calculated the volume of a vacuum silencer based on the maximum chamber pressure of the cartridge. By the time the bullet has traveled even 5” to the end of the barrel, the pressure inside the barrel is many times less. PV=NRT. Pressure X Volume = two constants I can’t remember X Temperature. Here’s a graph:
On firing, the cartridge reaches maximum chamber pressure when the bullet has only traveled a very short distance. That means a very small volume at high pressure (and temperature!). 5” away, when the bullet leaves the barrel, pressure is down to “only” 1400psi or thereabouts, and the volume is much greater. A bullet going down a barrel is very much like the piston in an engine going down it’s cylinder. So when the bullet exits the barrel we have a volume of gas equal to the volume of the barrel times the pressure of the gas at that point. This is much much less gas than the volume of the barrel times the maximum pressure achieved. So it wasn’t deliberate cheating, it was me just rushing along and not thinking things through.
What I tried to create was an actual silencer. Real silence. One that you could use to shoot a bullet so that it made no noise at all. No bang, no pop, no “foop” like on TV. The problem is that my idea can only AT BEST eliminate the noise of the firing. The bullet itself makes noise, as this famous graph points out:
The red line is about the maximum speed you want the bullet to go. Any faster and it gets loud.
I am certainly not an expert on silencer design. I’ve read no more than a dozen papers on them, and visited perhaps that many web sites. As far as I can tell mine is the only vacuum chamber design out there, because it just isn’t at all practical. But I was reaching for absolute silence.
Reader P. Beck provides links and points out significant efforts across the gun history time line. None of them though sought perfect silence. The best you can get is a gun that makes no more sound than the background noise, which is usually somewhere in the 75dB range or a bit louder. Keeping the bang under 95dB is quiet enough to not panic the neighbors. Under 105dB outdoors will hardly be noticed by people even 50 yards away. Well, they might notice, but they won’t assume gun fire. This is why a silenced firearm uses subsonic bullets, to add as little noise to the mix as possible.
Follow P’s link to the De Lisle carbine. This is about as good as it can get. They made a special gun that used a big slow bullet and used a huge suppressor with a great big expansion chamber. My bet is that they didn’t use regular military .45 ACP ammunition, even though they used regular military 230gr .45 ACP bullets and created nearly standard velocity. In a strong firearm you can use a slight charge of very fast gun powder, like 3.6gr Hodgdon Clays, to produce that kind of low velocity without generating much gas. It’s a sharp quick kick instead of a long slow push, and it creates more chamber pressure than normal, but you get the required velocity with very little pressure at the muzzle. And low muzzle pressure means less bang. Always. The Wiki article says the gun was effective to 200 yards with a maximum range of 400 yards. I doubt the 400 yards. A typical .451” 230gr FMJ RN bullet fired at the stated 850fps velocity will be slogging along at only 640fps at 400 yards, which may not be fast enough to even shoot through a cotton shirt, much less into the skin behind that shirt. Then again, maybe it is. I don’t have that answer but it wouldn’t be hard to find experimentally with some cloth, some ballistic gelatin, and an episode of Myth Busters. Plus the trajectory and the wind deflection at that range are just crazy. Using a very generous 10” vital height, which is about all you can get away with when people are your target (aim at the base of the neck, and if the bullet hits 10” high you still hit them in the head, and if the bullet hits 10” low you still hit them in the upper chest) gives this round a point blank range (the maximum distance you can aim and shoot and be assured of a hit without trying to figure out the range to the target) of only 150 yards. This means a 126 yard zero, and from there out to 200 yards your bullet has dropped almost 42” and been pushed 10” sideways by a gentle breeze. At 400 yards it’s much worse.
Ok, so now I’ll paste in my response to P. Beck. Thanks for giving me an interesting morning writing all this stuff out and thinking about it. Feel free to rebut as much as you want.
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All of the following is in response to P. Beck’s comments. This is not done to silence him (hur hur) but to enable a discussion.
“A 240 grain Sierra Match-King, launched at a m.v. of 980 fps, loses about 100 fps per 100 meters of travel and has more energy at 100 meters than a .44 magnum does at the muzzle.”
Since you didn’t mention caliber, but the guns in some of the links you provided are .308, I’ll have to assume that’s the one you’re talking about. This is a superb target bullet, with a BC of .712 - that means it is really aerodynamic - because it is very pointy and quite long and heavy for it’s diameter. My trajectory software shows that when shot at the subsonic speed of only 980fps it only slows down 27fps in the first 100 yards, and 24fps in the next 100 yards. BC - ballistic coefficient - is dynamic with respect to velocity. So is bullet drag. The faster you shoot a bullet, the faster it slows down, even though the highly aerodynamic ones slow down relatively less. Once you get below the speed of sound aerodynamics matters less and less; the slower you shoot a bullet the slower it slows down, and at the very slow end of things the shape doesn’t matter at all. But if you shot that same bullet at 3000fps, which is a realistic velocity for a serious magnum rifle cartridge, it would slow down 137fps in the first 100 yards. Fire it at about 2500fps and you do get the 100fps loss you suggested.
The above paragraph is stuff P. already knows, I just put it in for everyone else. But he is off a bit with the .44 Magnum comparison. The .44 Magnum often shoots bullets of the exact same weight, 240 grains, (larger .429” diameter though) but even in a pistol it pushes them out of the muzzle at 1300fps or faster. In a good rifle with hot ammo you can get nearly 2000fps. Since bullet energy is 1/2MV^2, the .44 Magnum has far more muzzle energy than this MK bullet fired at 980fps has at any range. I’ll cut P. a break - the .44 Special is a very similar but lesser pistol cartridge, and it shoots the same bullet at only about 900fps, which gives it less muzzle energy than this MK bullet has at even 300 yards.
“So. Heavy subsonic rounds. Trajectory like a rainbow? Yes. Unstable? No. ”
True, but only if you “cheat” a bit to assure stability. You need a custom built gun.
Let’s drag out the old Greenhill formula calculator, which is used to determine the minimum necessary rifling twist needed to stabilize projectiles flying in the air. P. probably has one on his PC like I do, and the rest of you can Google one up in a flash. There are thousands of them out there.
Just like throwing a football properly, you need to spin bullets around their longitudinal axis to keep them pointed in the proper direction. If they don’t spin enough they will wobble, and if they wobble they will not be accurate. The longer a bullet is for it’s diameter, the more it needs to be spun to be stable. This is what the rifling in the barrel of a gun does. It imparts a constant amount of spin to a bullet, one turn in a certain distance. Obviously this means that the faster the bullet travels, the faster it is spinning - often at 200-300,000rpm! - but the rate of spin doesn’t change. For every foot the bullet travels, it spins around a certain amount, regardless of it’s velocity.
Back to Greenhill. The .308 240 grain Sierra Match King is 1.6 inches long. That’s almost twice as long as the .308 150 grain Sierra SPRN (.895") which is a common woods hunting bullet in that caliber. Greenhill tells me that the MK bullet needs a 1:8 twist barrel to be stable at velocity under 2800fps and one of 1:10 for velocities over 2800fps, whereas the SPRN needs a 1:15 twist barrel for the low velocities and one with 1:19 for the high velocities. The vast majority of rifles use a twist rate of 1:9 or slower (click for examples), especially in the larger calibers, so in a normal rifle the MK bullet is not going to be stable. Sorry. It won’t even be stable in a regular .308 match barrel, which is usually a 1:10 or 1:12 twist. The same situation applies to every other bullet that is grossly heavy or super long for it’s caliber. Look what happened when the Army went to that heavy 70gr bullet for the 5.56 NATO. They had to rebarrel all their rifles with a 1:7 tube because that “big” bullet just wasn’t accurate in the slower twist that was perfectly fine for the older 55gr bullets. Hunters have had accuracy trouble with some of the Barnes copper bullets; copper is much less dense than lead, so an equal weight copper bullet is much longer than it’s lead counterpart, and therefore needs considerably faster rifling to be stable.
But you’d be silly not to use a custom barrel for a project like this one, so you can order one of those with whatever twist rate you specify. There is nothing wrong with spinning a bullet faster than the minimum amount necessary, but if you use too fast a rifling twist rate and shoot them at too high a velocity they will shred. Too much RPM just tears them apart because the metal isn’t strong enough, although copper bullets are much stronger than lead ones. So your custom fast twist barrel might just not work at all when you use smaller normal high velocity bullets in it. Everything’s a trade off.
Furthermore, I don’t think Greenhill goes far enough because it isn’t set up to go slow enough. I think there is a minimum RPM that a given bullet needs to be stable at any velocity. Come on, if you throw the thing, it will tumble. If you throw it like a football, it will tumble less, but the natural front to back imbalance of a pointed bullet will cause it to tumble regardless. Your custom barrel needed for those extra long bullets might have to have an even faster rifling twist to ensure stability at really low velocities.
I have made nearly silent ammunition for my .45-70. You take a big lead bullet, 400gr, and use 2 or 3 grains of Bullseye or Clays. It spits out the barrel at 500fps or less, and goes “foop”. The bullets tumble after 60 yards or so. A .45-70 usually has 1:20 rifling, which is fast enough for nearly all the blunt short bullets that rifle usually uses. So there is a minimum RPM and it’s not connected to velocity. Except that in a rifled barrel, it always is. Maybe you can locate a velocity dependent version of the Greenhill formula.
Ok, that addresses the stability issue.
On to trajectory and the ever fearsome ”rainbow”. Gravity is a constant. If you could take two bullets and set things up so that you shot one out of a level rifle at the same instant in time that you just let go of the other bullet right next to the barrel, both bullets would hit the ground at the same time, even if the one from the gun hit the ground 2000 yards away. So, the faster a bullet travels, the further it will go in a given slice of time, but if fired horizontally it will drop the same distance in that time slice as anything else. Slow bullets thus drop more relative to their horizontal distance traveled than do fast ones. This is what trajectory is all about. It’s always a rainbow curve: you have to shoot the bullet UP to get it to come DOWN on the target. The advantage of faster bullets is that they flatten out the rainbow quite a bit. The further away the target is and the slower the bullet is fired at and the less aerodynamic the bullet is, the steeper the rainbow curve of the bullet’s flight path will be when it hits the target. Hey, don’t forget that wind will have it’s say here as well. Google up that US Army test at Sandy Hook NJ from the late 19th century called “the .45-70 at two miles”. In that test, at such extreme ranges, the bullets were hitting the target nearly vertically.
What all this means is that to hit a target at a longer range with a slower bullet, you need to know exactly how far away the target is. Using P’s bullet and muzzle velocity as an example, the trajectory difference between 800 and 825 yards is more than the height of almost all men (6’ 6.5"), even though the bullet itself might still have enough energy to kill (350 lb/ft) at that range. His bullet takes almost 2 3/4 seconds to get to that 825 yard target, and our 10mph right angle gentle breeze is going to push the bullet more than 4 feet to the side. And I’ll be generous and ignore the difficulties involved in estimating how much he’ll have to lead his target if it’s walking along. So my point is that to hit anything at distance you are going to have to be really, really, REALLY good at range estimation: at 800 yards a 1% error will have the bullet strike 15” high or low, which is more than enough to cause a clean miss or to turn a kill shot into a minor wounding shot. Those nifty laser range finders you can buy are that accurate at best. Maybe the military has better gear, but I don’t think we can get our hands on it.
Lacking power? No.
Yes, you can kill people by throwing big rocks at them. There is a minimum velocity necessary for any bullet that allows it to penetrate. Each bullet shape will have it’s own number. My guess is that the pointier ones can penetrate at lower speeds than the blunter ones can, and I’d further a second guess that the heavier ones of a given shape and diameter can penetrate at lower speeds than the lighter ones of the same general shape and diameter. If a bullet doesn’t penetrate then it’s effect is similar to getting hit with a rock. A very small rock, because, hey, that’s all that bullets really are anyway, right? The “good news” is that people are really lousy animals, so a bullet that’s barely good enough to kill rabbits with is more than enough to kill people with. The 1000 lb/ft energy requirement some states have for deer hunting is far more than what’s needed for us. I don’t know what the human minimum is, but a wise hunter always uses more than what is minimally required.
In the Sandy Hook test I mentioned earlier, the bullets falling on the target at 2 miles range still had the power to penetrate several thicknesses of wood. But these were massive bullets, 500 grains of lead. And their trajectory was so extreme that it’s likely that gravity significantly increased their velocity as they fell downwards.
Inaccurate? Definitely not. “...one officer. before witnesses, shot the head off of a duck swimming on a nearby lake at 700 yards.” 200 yards.
This is an interesting statement. Given that a duck’s head is about an inch and a half tall, and about the same length excluding the beak, what we’re talking about here is accuracy within 3/4 MOA.
MOA is “minute of angle”, a term used to describe the precision - the repeatability - of the accuracy of a firearm. If you shoot 10 rounds at a distant small target, how closely together do the bullets group on the target? That’s precision. Generally, 1 inch at 100 yards is considered to be a real world Minute of Angle, though scientifically it’s a tiny bit different from that. Think of the bullets flying from the gun to the target, with whatever their amount of dispersion is, and you’ll quickly realize that a cone shape covers that dispersion. Granted, when you add the “rainbow” arc of the trajectory into the discussion the cone shape becomes bent, like one of those Bugles brand snack chips. But a cone still covers it, and if you remember your triangle math, you know that the longer the cone is, the wider the large end becomes, even though the angle between the sides stays the same. This means that while 1 MOA at 100 yards is 1”, 1 MOA at 200 yards is 2”, 3” at 300, etc. A firearm that can shoot groups of 1 MOA is more than just accurate, it is nearly the holy grail of firearms design. These guns exist, and modern precision manufacture has made them almost common, but it takes highly uncommon shooting skills to be able to shoot that well.
To hit that duck’s head more than just once, the shooter would have had to shoot groups at least as tight as 3/4 MOA. This is the kind of precision shooting that wins competitions between world class experts shooting off the bench with those machine-like rifles in the sport of benchrest. Shooting offhand with a normal rifle it is flat out miraculous. Let’s not forget that bullets are subject to wind deflection, barrel vibration during firing, bits of dust in the air that they contact while flying, and even their own slight internal instability caused by the most minute variations in the density of the metal alloys that they are made from, etc.
The slower a bullet travels, the more time it takes to get to the target, and the more time wind has to push it around. A gentle breeze of only 10mph is air moving at 14.6fps; if a bullet takes only 0.10 seconds to reach the target, and that wind is blowing at perfect right angles to the target, the breeze will push it to the side 17 1/2 inches. If the bullet is shooting directly into the wind it will slow down more than it would in dead air, and drop more. If the wind is directly behind the bullet it won’t slow down as much as expected, drop less, and hit high. That takes care of just 4 of the 360 angles; any of the other directions will be a “rise over the run” calculation like in Physics 1. And all of this assumes that the breeze is constant, which it never really is. Wind gusts, all the time.
What I’m saying here is that hitting that duck was pure luck, combined with excellent shooter skill a) at reading the wind and b) with extreme familiarity with the trajectory of that particular firearm and it’s bullet. Great shot, but even David Tubb would have a helluva time doing it twice. (Mr. Tubb is just about the greatest marksman on earth)
GET ME OUT OF HERE ...
Oh, and here's some kind of visitor flag counter thingy. Hey, all the cool blogs have one, so I should too. The Visitors Online thingy up at the top doesn't count anything, but it looks neat. It had better, since I paid actual money for it.