Should it really be all about Momentum?

[Molon Labe]

I have seen some huge recoil equations! If you really want to get accurate about recoil you need to factor in the weight of the powder because it to is mass that leaves the barrel.

Like you and Deavis said, moving gas + moving powder will also have momentum (and thus affect recoil), but trying to quantify it is very difficult. A lot of the chemical energy is turned into heat, which does not affect recoil... gas + powder travel in a variety of directions, which will add/subtract to recoil... the mass of a moving particle may change over time... a particle may get lodged in the barrel instead of exiting the barrel... etc. It's impossible to measure these variables to any degree on accuracy, so analytical models must be designed (e.g. FEA) and approximations must be made. For the rest of us blokes, the easiest thing is to simply measure the bullet's momentum, and tack on a few percent for the momentum of the gas & powder.

And it gets even more complicated. Two guns could have the same momentum during recoil, but one gun could be high mass * low velocity, and the other gun could be low mass * high velocity. Again, same momentum. But would the recoil be exactly the same? I don't know, but my guess would be "no."

 

[lbmii]

Regardless, we are not trying to land on the moon.

 

[Yooper]

I believe that penetration is the only practical value to be sought, after all, what do we want the bullet to do when it hits the target other than penetrate? I think it may boil down to sectional density and velocity as determinants in projectile potential, regardless of caliber, and penetration is the net result. For a (theoretically) non-expanding bullet there will be an optimum velocity beyond which velocity increases show a diminishing return, but will always increase (though slightly) as velocities increase. Greater sectional densities will yield higher optimum velocities. For expanding bullets, there also will exist an optimum velocity for proper expansion and when velocities increase beyond this point, penetration will actually decrease due to the accelerated expansion. An expanding bullet is effectively changing its sectional density as it expands. All of this is relative to the density and makeup of the medium the bullet is penetrating. We already have the figures for one medium, air, through ballistic tables. As the density increases, the penetration decreases dramatically. Wet newspapers, ballistic gelatin, etc. all have their own characteristics, but none are as diverse as, say, a game animal. An animal has a very elastic skin, muscle tissue, bone, cartilage, internal organs, various fluids. etc. As far as "knock-down" potential or "killing power" are concerned, I don't think they exist. The mass of a bullet is so minute compared with most living targets that it can't "move" the larger mass effectively.

 

[Archangel]

Terminal ballistics are affected by a lot of factors. Momentum is one, but it is certainly not the only factor at play.

Take for example a 230 gr .45 moving at 850 fps (momentum = .87) vs a 55 gr .223 moving at 3000 fps (momentum = .73). The .45 has a greater momentum, but the .223 has unquestionably greater terminal ballistics.

The ideal bullet (as far as wounding) will expand to as great a frontal area as possible, and penetrate as deep as possible without exiting the body (which would just waste valuable energy / momentum / impulse / jules / kilowatt hours / etc). This gives you the largest possible wound volume, aka largest area to bleed out from, and greatest chance of hitting somethign vital. And if that wound volume can be broken up into multiple would tracks, giving you an even greater likelyhood of hitting something vital (al la the .223), so much the better, so long as the individual wound tracks still penetrate deep enough to hit said vital sonthings.

Now, a heavier bullet in a given caliber tends to penetrate deeper than lighter bullets, but also tend to be slower. Velocity is one of the main factors in expansion, so you don't want to go too slow and heavy. Bullet shape is also a factor, since round noses tend to slip through flesh, leaving a sub caliber wound track. A flatter, or sharp shouldered profile will crush / cut the flesh, creating a wider, full caliber hole. Of course, that also creates more drag on the bullet, slowing it down faster and decreasing your penetration. Then you have factors like temporary stretch cavity and fragmentation, just to complicate the matter even further.

Basically, to make a long story longer , there is a "sweet spot" in the trade off between bullet weight and velocity that gives you an optimal combination of penetration and expansion for your given bullet design and caliber, while maintaining a safe operating chamber pressure in your cartridge.

I think you'll probably find that the reason why the rounds you are looking at fall in the same range momentum-wise has more to do with keeping chamber pressures safe than it does with a momentum-based design criteria.

And if anybody can come up with a physics equation to say all that, my hats off to you. I'm not even going to try.

 

[Dave R]

I'm going to attempt to simplify this. Its simple in my simple little mind, anyway.

If you are worried about penetration of big/thick game, use momentum.

If you don't care about penetration, but only about explosive, shallow wounds (like varmiting), use energy.

I believe that penetration is the only practical value to be sought, after all, what do we want the bullet to do when it hits the target other than penetrate?

I disagree.

FWIW, penetration beyond the target = wasted momentum.

Bingo!

If you're shooting Prairie Dogs, penetration is of little practical concern. Gimme energy, baby!

If you're shooting Cape Buffalo, you want as much penetration as you can get.

For deer, or self-defense, you want something in the middle.

For self defense purposes, a human target is relatively thin-skinned (give or take a few NFL linemen or equivalent.) So you don't need more than about 12" of penetration. That's why the FBI set its specifications for ammo the way they did. They only want enough penetration to get to the vitals, and they don't want a lot of penetration beyond that. That's why they like expanding bullets that disrupt as much tissue as possible within 12".

That's the way I see it, anyway.

 

[Jim K]

Hi, Deavis and guys,

Back for a moment to recoil. Deavis wrote:

"The felt recoil is dependent on many factors, not just the momentum of the bullet. You do not need a moving bullet to get recoil in a recoil operated gun since there is more than one degree of freedom in the system. You can get the same effect by blocking the barrel and making a very small hole in the end of it with a bolt action rifle. It won't be the normal "recoil" force as you think of it but it will be recoil."

Sorry, but in normal operation, you DO need a moving bullet to get recoil. If you block the barrel so the bullet cannot move, there is NO recoil. None. This has been done, and (in spite of misleading information in a popular book on the 1911 pistol) it really does take a moving bullet to cause the "opposite effect", recoil. Gas pressure is not a factor, since it operates equally in all directions in the chamber; the net effect is zero. There is no recoil, no movement of the slide or barrel, and the breech remains locked.

Now if a barrel is partially blocked so that only a small opening is left, and a cartridge with no bullet is fired, the gas generated by the burning powder escapes through the opening. Since that gas has mass, there will be recoil in the opposite direction. If that were kept up continuously, the rifle would continue to move backward. In fact, it is just exactly that, on a much larger scale, that causes a rocket to take off. The escaping gas, a low mass but one moving at very high speed, escapes out the bottom of the rocket, causing it to ascend from the recoil. That is why a rocket does not need air to function; it works on recoil.

So it would be perfectly accurate to state that recoil put men on the moon, and recoil brought them home.

Jim

 

[FNFiveSeven]

Felt recoil is proportional, usually, to recoil energy. Not including the effects of a muzzle brake, the recoil energy is = ((Mass of bullet+ Mass of powder)^2 * (Muzzle Velocity)^2)/Mass of the Gun.

 

[Yooper]

Dave R

Explosive effects on varmints are also the result of bullet penetration. The bullet must encounter resistance to expand and unless it simply blows up on the surface, it is penetrating the target. It is still about penetration, either more or less.

 

[Dave R]

You're right, Yooper. You need about 2 inches of penetration for varmits.

Maybe 4" for Coyotes.

 

[Molon Labe]

I just did a google search on the following keywords: bullet terminal ballistics energy equation. I started reading some of the links, but quickly became disenchanted when I saw some gross errors. (For the millionth time, the kinetic energy imparted to the gun during recoil does not equal the kinetic energy of the bullet!!!! The bullet has a lot more kinetic energy.) I have Robert Rinker's book "Understanding Firearm Ballistics" at home. I'll look through it when I have time.

 

[Yooper]

Molon Labe is correct. 250 gr. bullet, 44 mag, 1400 fps, 3 lb. S&W 629. Bullet: 1088 ft/lbs. kinetic energy, Revolver: 21.55 ft/lbs kinetic energy.

 

[GunnyBob]

http://www.thudscave.com/npaa/articles/howhard.htm

Interesting page titled: How Hard Does It Hit.

Descriptions of Kinetic Energy, Momentum, and Sectional Density.

 

[Deavis]

Jim,

I think that you are not defining recoil in the same way that I am and in addition you are failing to standardize your argument. This argument depends on the type of action you are using. Let me state these facts as they apply to recoil. Recoil is any movement of the weapon that can be felt by the operator.

1) In a gun that has a locked breech that is not recoil/blowback or gas operated, there is no recoil if you if you block the barrel and pull the trigger if certain conditions are met. As long as the lugs and barrel are stong enough to hold that pressure, nothing happens, you are correct in this instance. If the lugs shear or the barrel bursts, you will enjoy recoil as the gas quickly forces its way out. The faster the gas leaves, the more you will feel it.

2) In a gun that has a locked breech that is gas operated, the gun will have a recoil and cycle the action if the barrel is blocked and sufficient gas is introduced into the system to unlock the breech. This how a blank gun works; you plug it, shoot it, and watch it cycle. The felt recoil will be due to the cycling of the action which is going to either short cycle or over cycle the action. There is 1 single point that could result in a flawless cycling that would be transparent.

3) In a blowback/recoil operated gun, if the barrel is blocked and the gun is fired, there will be recoil if sufficient gas is produced to unlock the breech from the barrel.

Let me disect your argument...

If you block the barrel so the bullet cannot move, there is NO recoil.

That is only true for case 1. You can remove the bullet if you want, you don't need it in any of these cases. Crimp the end of the catridge and call it a blank.

In case 2 and 3...

Gas pressure is not a factor, since it operates equally in all directions in the chamber;the net effect is zero. There is no recoil, no movement of the slide or barrel, and the breech remains locked.

In cases 2 and 3, this is incorrect. The pressure is indeed acting in all directions in the chamber, except that it isn't confined to the chamber. If you have a bullet in there, it will fly to the end of the barrel and stick there (bulging the barrel where it sticks). The gas will then be operating on all surfaces equally. However in case

2) The gas will seek an area of low pressure (take a M16) and go into the gas tube. It will then operate on the bolt carrier with the same force it is applying to the barrel, the barrel obstruction, the chamber, etc... If the pressure is big enough, it will operate that action. If not, your gun will get hot and eventually cool off. This is exactly how that gun works in the first place. You simply need a bullet to keep the pressure in the vessel (right after it passes the gas tube hole) as the gas quickly fills the tube and operates the action.

3) The gas will, once again, create equal pressure on all the surfaces it can. In the case of the breech face, it will push back against it via the case head and if the pressure is enough, it will unlock the breech and cycle the action. Just like if you grab the slide and pull it back, that gas can force it open. The breech is a degree of freedom in the system, just like that operating rod is.

If cases 2 and 3 were not true, then pneumatic cylinders wouldn't work. You squeeze fluid into a cylinder under pressure, and if something can move, it does. As a matter of fact, it is a *really* efficient way of moving something. We can easily take our guns and turn them into pressure vessels. So here they are

1) A gas cylinder with an inlet but no outlet
2) A gas cylinder with an inlet and an outlet piping with valve that is operated by a rod, with a piston facing the gas, held in place by a spring.
3) A gas cylinder with a simple pop off valve held down by a spring.

Pump in extra gas and #1 explodes. #2 will eventually overcome the the spring and open the valve. #3 will pop once the pressure overcomes the spring. This is exatly how huge gas compressors work. Their pistons have springs inside of them that give when a certain pressure has been reached so that gas can vent by them.

and a cartridge with no bullet is fired, the gas generated by the burning powder escapes through the opening. Since that gas has mass, there will be recoil in the opposite direction.

That is exactly the principle I have been trying to get across. I don't know why you recognize it there but don't recognize it in cases 2 and 3. The fact of the matter is that guns have been created that don't require a bullet moving to cycle the action via blowback/recoil or direct gas. There is an instance where a 50 cal machine gun was cycled with a 22 bullet via gas pressure in the chamber. No way in hell the quantity of gas from the 22 could operate it in the normal gas tube fashion, but by using pressure in a very wise way it was done. The original Garand operated on the recoil of the primer! It moved a couple hundreths of an inch as the pressure of the burning powder forced it back in the cup.

You can prove this is true if you have a threaded barrel. Put the gun in a vise, thread a pressure fitting on there, crank it up to 35,000 PSI, open the valve and watch your 1911 action operate.

* edited: It's a gun, not a bun!