Home Forums Hunting & Shooting Ask The Gunwriters Recoil and short barrels

OK, so we've established that a projectile leaves the muzzle of a barrel with energy and momentum. The energy is kinetic and the momentum carries impulse potential. Hawkins likes pounds-second for impulse, sounds cool that way.

We've also agreed that the gas from the powder charge likewise moves out with energy and impulse potential. Each has contributed to recoil. Both are in the recoil energy formula, by the way.

What wasted all the ink in this thread was not the obvious statement above but how it relates to the original question! The original question, poor guy, he just wanted to know about shortening a rifle. Deersmeller really articulated his question well.

Professional opinion was obtained from Mule Deer even before I entered the discussion. I just wanted to post the recoil energy formula I was using on my Excell spread sheet. Mule Deer opined that it would be a wash based on his observations and experiments. Free hanging/ recoiling rifles, what more could you ask for. I think he's right.

You can calculate impulse values all you like but BOTH barrel lengths have correlating gas and bullet recoil contribution. I see it like this, the chemical energy from the powder goes into making the bullet and gas accelerate down the barrel, with each inch more and more energy goes from gas to the bullet. One guy said, what if the barrel was so long that the bullet ended up stopping inside, or better, the length was equal to the point at which the bullet started slowing down. Well, in that case, all the chemical energy ended up in bullet movement and waste heat.

I believe it�s �mostly� six of one or half dozen of another. More bullet energy + less gas energy (approximately equals) less bullet energy + more gas energy. A shorter barrel adds no form of energy, impulse or force so there can be no additional recoil. This is conservation of momentum and energy pure and simple. That�s the final opinion of a non-practicing mechanical engineer.

We even gave reasons why there is the percieved recoil increase when shortening a barrel. Both objective and subjective.

Lastly, I think the term �Rocket Effect� is misleading to readers when talking about short barrels. More gas energy coming out of the barrel should be called just that. The "rocket" is the gas energy interacting with the surrounding air. As the gas comes out it begins to impart force which is what we call recoil. It does this with long and short barrels, just in differing degrees.

gmack

Oh come on.

Blow up a balloon and let go of the balloon. Rocket Effect!

It's not so hard. Pressure is pushing evenly on all sides except for the side across from the hole. More pressure is pushing on that side so the balloon goes that way! Rocket Effect!

Once the bullet leaves the barrel, Rocket Effect!

No, I do not think Rocket Effect is misleading. It sounds like what we are talking about here. Recoil do to rocket effect!

If you tape a long straw to the balloon will it take longer for the air to leave the balloon then it would if a short straw was used? If so, a longer barrel would have less thrust in its Rocket Effect. How much? I don't know. That is not my strong point. All I can saw is; It will take longer.

The same goes for the hole in the straw. .224 or .308. Or lets try .308 or .243. Which will have more recoil with the same pressure.

Surely, you jest.

For what it's worth, I had QuickLOAD run recoil numbers for a 7 lb .308 with a 180 gn bullet in both 28 and 16 inch barrels. It shows lower recoil for the short barrel, but not by much, about 10%. The free recoil numbers are 13.5 fps for the long barrel and 12.3 for the short barrel. At muzzle exit, the free recoil numbers are 10.9 fps for the long barrel and 9.7 for the shorty.

I do not know how good QuickLOAD's numbers are for the rocket effect. That the gas effect is about 2.6 fps for both could suggest the author used a simple estimate for the total gas effect. However, the computed duration of the gas effect is about 40% shorter for the stubby barrel, and the computed rocket effect isn't quite the same, 2.67 fps for the shorty and 2.55 for the long. This suggests a more complicated estimation of the gas effect is being computed. It may also involve the percentage of the charge it computes as having burned, being lower for the 16" barrel. Unfortunately, I don't know where my manual for QuickLOAD is, so I can't look up how he computed this.

I certainly would have guessed the rocket effect is greater with the short barrel, but supersonic flow is tricky stuff and can be non-intuitive. When the bullet exits at 16", all the gases are hotter than at 28", and this may allow for a more rapid (briefer impulse) blow down; that would be consistent with the shorter blow down time QL reports. It would also suggest a faster flow (more momentum), but QL didn't figure so. I do recall such flow can be choked at the muzzle, and this might put a limit on the average gas velocity, which could explain the fixed recoil effect of the "rocket". Beats me. Perhaps QL has a bug here?

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OK, I thought about QL's numbers further, and I don't think they are too far off.

While QL predicts the hotter, higher pressure gases in the 16" barrel blows out in considerably less time, I was forgetting that there is far more volume to evacuate with the 28" barrel. QL predicts a blow down time 1.83 times as long for the 28" barrel, but the volume to be emptied is 1.70 times as great. This suggests QL figures the flow at the muzzle is choked. (Keep in mind gas trying to escape from the chamber has 75% farther to travel in the long barrel.)

Another not unrelated point: in the long barrel, the 13% higher bullet muzzle velocity implies the average velocity of the gases is already 13% higher to begin with when the blow down begins. They don't need to be accelerated as much to end up with the same rocket effect.

Here in Germany it is basic hunters knowledge (and part of the written test hunters apprentices have to do) that "rocket effect" adds circa 20/40 % to formula derived RV/RE values.
The more powder to burn, the shorter the barrel, the higher the gas particles velocities, the more "backthrust" to the breech, the more recoil.

Walter LAMPEL (Technical President at RWS N�rnberg then) in the 1940s already did the according pendulum measurements.

In real life, "wrong" stock dimensions like short LOP or too much drop or a small or (with low mounted scope on a stock too straight) not fitting recoil area, as well as "stock crawling" and "check weld", as well as powder reaction as well as muzzle blast and muzzle fire do add to "felt recoil", of course.

I did my own measurements with a "recoil sled" in the early 1960s and found out that loadings of appr. same muzzle energy production showed about the same "sled" measured recoil.

So A) a high velocity lightweight bullet load in 6,5x68 and B) a 8x57 IS load with double that bullet weight while at lower velocity - same energy yield - produced about the same (measured) recoil when anything (rifle, scope, weight) was equal.
"Felt" recoil was less with the 6,5x68 only when shooting this load with ear mufflers (which were not so common these old times).
Compared to RE, RV is contributing much more to felt recoil.

RD

More of my diatribe that I would like to share that helped me form my understanding of this topic:

Recoil ENDS �virtually� the moment the gas and bullet leave the end of the muzzle. The �rocket effect� is MOSTLY the effect of the gas movement INSIDE the barrel. The source of my scorn of the rocket term is that it implies otherwise.

YES, gas leaves with SOME potential energy in the form of pressure that is dissipated in close enough proximity to the end of the barrel to add a SLIGHT thrust from outside the barrel. This thrust would be equal to the cross sectional area of the end of the barrel times whatever pressure specter was around. The effect difference between barrel lengths AOTBE is small!

The one guy thought the coned suppressor on his Enfield added to recoil. I can see that this would have the opposite effect of a muzzle brake by converting more exiting pressure to thrust directed back to the shooter; more than a square end. Good example for learning!

You think a balloon is a good example of rifle gas leaving a barel? Try popping the balloon, now how far do the fragments go? This is a better example of the effect of exiting gas energy from pressure, in MY opinion, for a gun; a small fraction of the energy goes to work on the gun.

Once the mass(s) moves out, LEAVES, it has momentum that will pass on impulse force to anything it runs into. In the case of gas, this is what you SEE that I think confuse some. Bullets punching through steel plate or gas pushing debris, it�s the same effect. This is not recoil.

I think, perhaps, part of the boresighting possibility can be explained by the fact that the bullet also begins to drop the instant it leaves the muzzle.

The friction issue is a non-issue. True it may reduce the rate of acceleration of a bullet but as long as the bullet is accelerating, noy deccelerating, the friction in the bore matters not in terms of recoil effect.

My own take on recoil is that it is a perceived thing at least as much as a measurable deal. I do think one of the factors in felt recoil can be the arcing swing (or not) of the barrel. The longer the barrel, the more the mass swinging and being further from the center of the arc, it becomes disproportionately greater in terms of perceived effect. (It may be a wrongly applied example but it seems to me that a hard recoiling long barreled revolver is more likely to do the "cranium tap" than is the shorter barreled one. A rifle exhibits the same effect but to a lesser degree, I think.)

Popping a balloon would be a good example of sudden loss of pressure. If memory serves me right the balloon will unwrap and fall to the floor. In this case I would me more intrested in what happens to the air then the balloon.

This is not religion, beliefs don't count.

The way I see it there are alot of forces at work here. Has anyone kept track.

Thank you for your input ROE_DEER.

gmac; Sir you have me at an advantage. You studied Physics
In the late 70's. My under graduate work was done in the 50's. It's breathtaking how much has changed.
Good luck!

Here's just one more thought...

If 4,000 fps is a reasonable estimate of the AVERAGE speed of the gas after the bullet exits, it's a reasonable guess that it's much higher than that to begin with. So, maybe the rocket effect happens quickly enough to provide the highest force. Or, maybe not. The measurements will tell us.

This is for me, I like hearing myself ramble like a little kid telling all they think they know. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

I believe the key to understanding this topic is accepting that the end of a barrel is a transition point where recoil stops with the mass(s) departure; OK almost. Bullet yes, gas mostly IMnotsoHO.

The gas exits with pressure, velocity and heat. And it exits from a shorter barrel with more pressure, velocity and heat. Gas at 4,000, 5,000, 6,000 fps or more, whatever, simply serves to show that chemical (potential) energy has been morphed into kinetic energy. Experimentation will show that gas momentum is no longer acting on the barrel and is moving away.

The pressure of the exiting gas tells us that not all of the powder�s chemical energy was efficiently used. Some that was converted to pressure to drive the bullet leaves the barrel as waste potential energy. Yes, AOTBE, if the bullet has less kinetic energy (velocity) then the gas exits with more kinetic, potential, and heat energy. This pressure probably lingers long enough to kick back.

I have learned along the way and now see that this pressure, I have succumbed to use of the word rocket, is the �second� stage of the rocket; the first being the ongoings inside the barrel. I think my biggest disagreement with others in this thread is how much applicable thrust is in this second stage.

I think one of the functions of the cone you see on a �real� rocket is to utilize this potential that would otherwise largely be lost. As the pressure dissipates in all directions a vector of force is captured by the cone surface and sent back for propulsion. It�s the magnitude of this force that I believe is small, especially without a funnel.

I�ll admit that in the beginning I didn�t understand the two stage rocket effect. I believe the second stage is like a blow-back. The pressure flashes into kinetic movement. But look at what it�s acting on, the end of a barrel. That�s not much surface area to act on and the velocity keeps the charged gas moving away!

The actual measurements will tell us what the engineering has already told us, And by the way, Bullet friction merely robs the system of energy. Heat is drawn away, making less energy available to push the bullet. Friction does raise pressure but this mostly ends up as wasted potential out the end of the barrel. Recoil is reduced a wee bit also.

gmack

Look at the balloon. You blow it up. As you hold onto it or tie a knot in it, all the energy is going in an outward direction. The force is even in all directions. We have potential energy. untie the end and let it go. Inside the balloon pressure is the same all the way around the balloon except for the end where the air is going out and the opposite side of tha balloon. The faster air moves the less pressure it has ( Like the wing on an aircraft) and on the side with the hole air is moving faster then on the opposed side. So we now have more pressure on one side of the balloon and the balloon goes in that direction. The air outside of the balloon doesn't propel it. The pressure inside the balloon propels it.

As for the cone. When you expand gasses in a cone, the gasses try to expand in all direction but the cone gets in the way. So the gasses expanding in the direction of the open end of the cone expand and move. This will make lower pressure. We now have a rocket.

If you remember. the cone on the Enfield was used before the V2 came around.

The moment the bullet leaves the end of the barrel or uncorks the barrel, we have gasses expanding and moving at very high speeds in one direction. Out. It is only for a split second but it is there.

Muddog,

If your balloon analogy was fitting it would mean my therory would be wrong. The problem with it is that compressed gas energy is a different form of potential energy than chemical energy. Simply put, uncorking a balloon is not the same as what is going on when a bullet is pushed out a barrel by a changing state of matter.

I was prepared with an answer cause I knew someone would bring it up again. Nice try. Thanks for reading my post <img src="/ubbthreads/images/graemlins/cool.gif" alt="" />

gmack

You mean it is more like a solid fuel rocket? Same differance.

Ok, I'm out of here! I'll check back latter to see what kind of numbers denton comes up with.

I'm sorry if I steped on toes.

I want to correct myself. The potential energy in a rubber balloon is mostly stored in the elastic material, not compressed gas. The compressed gas was the scuba tank. It's still not apples to apples with gun powder.

I promise not to go back and correct all my failures to be exact. I'm thinking about the one about there being no conservation of momentum in the universe, I meant because of friction there's no true conservation. The same could be said of energy, you know because of the expanding universe thing.


gmack

Let see, recoil is the opposite reaction to the bullets action going in the other direction. (for each action there is an equal and opposite reaction, physics 101). The reaction is the opposite to the action of a bullet and its velocity (mass x acceleration, physics 101), Reduce either and the action will be lessened. Lets assume that you fire a factory cartridge in a gun with a 18 inch barrel and one in a 24 inch barrel, same powder charge, same bullet wgt. The velocity in the shorter barrel will less than in the longer barrel therefore lessening the action which produces the reaction, recoil. Exciting gases have almost no mass so contribute virtualy nothing to the reaction. But, the shorter barrel has less wgt to absorb recoil. The velocity is higher in the longer barrel but it has more wgt to offset the difference. To say there is significant difference in recoil of the same rifles with different barrel lengths is kind of more a feel thing than anything else. I had for years shot a 7x57 custom rifle with a 18 1/2 inch barrel as my hunting rifle. 2 yrs ago I had it rebarreled with a 24 inch barrel. I cannot tell there is any difference in the felt recoil.

Actually, it should be a wash since the short barrel will produce less velocity but have higher exiting bullet pressure. Conversely, the longer barrel will have lower gas induced recoil but will be acted upon longer by the gases and produce a higher bullet velocity. This will mean more momentum induced recoil but less gas recoil and will be nearly equal.

The P101 idea of equal and opposite reactions isn't wrong but it is easily misunderstood, misapplied, or not fully accounted for in detail. By some reckoning it would seem, a gun with a long enough barrel should have the greatest recoil potential since it will produce the highest velocity but, if long enough, the same barrel will also slow the bullet down until it comes to a complete stop due to friction. Would this gun have recoil? Yes, and it would be in both directions in equivalent amounts though durations would likely be different, but the gas "rocket" effect would be nil.

"Exciting gases have almost no mass so contribute virtualy nothing to the reaction"

Isnt mass conserved in physics 101 too? <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> Dont have a textbook handy, but the mass of the gas should be about the same as the mass of the powder charge.


My vote is for less recoil with the short barrel, but i dont think it would be noticable. But the muzzle blast would be.

Let's suppose that we have two rifles of the same weight that are identical in every way except barrel length. As soon as the cartridge fires, the bullet is pushed forward and the breech is pushed rearward.

The work done by the propellant gases is the force that they push on the bullet times the distance that they push. If the barrel is longer, then the gases do more work pushing the bullet. Likewise, the longer barrel means that more work is done to push the shooter rearward.

We often hear that the propellant gases travel at half the speed of the bullet. That's not exactly true. What is meant by this is that one end of the gas column moves at the speed of the bullet, while the bullet remains in the barrel, and that the other end stays put at the rear of the cartridge case. Therefore the center of mass of the gas column does move at one half the speed of the bullet, while the bullet remains in the barrel.

When the bullet leaves the barrel, the gas column is no longer constrained. Like a third grader fleeing school when the bell rings, when the bullet leaves the muzzle, the gas roars out, passing the bullet. In fact, the gas leaving the muzzle soon forms a cloud that looks like a giant onion.

As the gas leaves the muzzle it does so at supersonic speed. In the first foot or so from the muzzle, the gas takes the shape of a cone, or funnel. As the gas races both forward and outward, it acts like a rocket's exhaust, further pushing the muzzle (and shooter) rearward.

Since the gas leaving a shorter barrel is at a higher pressure than the gas leaving a longer barrel, this effect is accentuated. In practice, the shorter barrel would probably produce more recoil than the longer barrel. However, as other folks have pointed out, the louder muzzle blast of the shorter barrel would likely increase the perception of recoil far more than any real increase.

engineer