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

That type cartridge does exist. A lot of work on it came from a couple of fellows that work at Thiokol. A friend of mine is also a writer, and he has a rifle chambered for the cartridge, on loan from the manufacturer.

Actually, it is. We even have most of the needed numbers to plug into the principles.

A 180 grain bullet accelerated to 2700 fps imparts 180/7000/32 * 2700 = 2.16 slug-feet/s of momentum to the rifle, over a period of about 1.5 milliseconds. You can quickly run the calculations for a shorter barrel, with a loss of 100 fps. It comes out 2.089 slug-feet/s, a negligible difference.

A 60 grain charge of powder propelling the bullet will have its center of mass at mid-barrel just before the bullet exits. So it will impart 60/7000/32 * 2700/2 = .36 slug-feet/s of momentum to the rifle, during the same 1.5 milliseconds.

Using Scott's approximation, when the bullet exits, and the gas expands freely, it will impart another 60/7000/32 * 4000 = 1.07 slug-feet/s of momentum to the rifle.

The only remaining issue is how quickly that last 1.07 slug-feet/s of momentum is deposited. If it happens in much less than 1 millisecond, then peak force may be much higher than during bullet acceleration. Or, the reverse may be true. That's really all we have left to find out.

The principles all work, and we have all the numbers but one. The one thing left to measure is how zippy the discharge of gas is. Well, it would also be nice to know how fast momentum is being deposited during the first few inches of bullet travel, too. Perceived recoil is probably proportional to peak force.

Whatever the results of the measurements are, I'll post them here.

*NOTE: A slug of mass is the mass of an object that weighs 1 pound at the Earth's surface. If someone shoots a slug at you, duck.

No we haven't. To put things in perspective, suppose I have a 9.3 BS for which I have a load worked out for that hypothetically delivers a 286 gr. NP at 2400fps with a load of 65 gr of a certain powder. Then by Scott Mayer's formula we have a total momentum of 1000*946.4 gr.ft/sec using a �VC� of 4000ft/sec. Now suppose We use that same load in another 9.3 BS which delivers a muzzle velocity of 2300 fps for no other reason than that it has a shorter barrel. The velocity of the bullet has decreased by 4.16% but in order for the rifle to recoil with the same momentum, VC must increase by 11.1 % . That is, VC must increase by more than 11.1 % for the rifle to actually have more recoil. Could that really be the case?, well maybe, but nothing we have hashed over here would indicate that it is so.

Hi Denton, I have to disagree with the two points I have underlined.

- I do not see the rationale behind these 2 elements of recoil imparted by the gas. One element - with the right average speed of gases - is all we need.
- Where does the 4000 come from ? As far as I know, this figure of 4000 ft/s for the speed of gases (average ? peak ? ...) has never been validated in a regular barrel let alone in a shortened one. Besides the exhaust speed depends on many variables (load, caliber, powder, temperature, mass of bullet, ... and of course length of barrel ). Therefore 4000 ft/s is largely an arbitrary figure. This approximation may be useful for certain evaluations but it does not work here.

The difficulty of the question comes from the difficulty of knowing the real effect of the gases. This includes questions on the speed/mass profile of the exiting gases, allowing for consideration of the still unburnt powder at the moment of exhaust.

These issues cannot be solved without serious physics investigation (qualitative and/or quantitative), estimations are not enough.

This is one reason I very much look forward to the results of the quantified experiments you propose to conduct.

Thanks for your serious interest in the question I proposed.

[/quote]

*NOTE: A slug of mass is the mass of an object that weighs 1 pound at the Earth's surface. If someone shoots a slug at you, duck. [/quote]

"the slug is a mass 32.2 times larger than the mass of the standard pound". This is what my book says.

This means a pound has a mass of 1/32.2 slug on earth.

Just keeping it real. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

I believe Hatcher was speaking of gas effect WHEN ALL ELSE IS EQUAL: muzzle velocity, rifle weight, etc.

If muzzle velocity of the bullet has no appreciable effect, why does a .300 magnum kick more than a .30-06, if both are loaded with 180-grain bullets and 60 grains of powder, and the .300 gets 200 fps more velocity? (This can be done, by using a slow-burning powder in the .30-06 and a faster-burning powder in the .300 magnum.)

The 100 fps lost when a barrel is shortened 4" does indeed make a difference in recoil. Otherwise Newton was full of it.

My guess, both from experience with many rifles and some previous experiments with free-hanging rifles, is:

1) Cartridges that have relatively high bore/capacity ratios, shooting light bullets, will show more recoil from rocket effect.

2) Cartridges that have relatively low bore/capacity ratios, shooting heavier bullets, will show more recoil effect from bullet weight.

All of this was obliquely demonstrated some years ago when I tested two rifles with removeable muzzle brakes, a .375 Weatherby and a .458 Winchester. Both weighed about the same, used about the same amount of powder, and recoiled the same without the brake. But the .375's recoil (as measured by rifle movement) was reduced about 40% by the brake, while the .458's was only reduced about 20%.

This was because the .375's muzzle pressure was far higher, due to its higher capacity/bore ratio. This translated into more velocity for the gas going through the muzzle brake, and less rearward push.

The .458, I must report, still pretty much kicked like hell.

All of this implies that rifles chambered for cartridges with high capacity/bore ratios will experience more effect from having the barrel shortened than will other rifles, chambered for cartridges with low capacity/bore ratios.

MD

I wasn't even going to try to make sense of Hatcher but I believe you are correct. He must have been talking about loading the bullet back to the speed measured before shortening the barrel. Congratulations. There's no pseudo science after all!

I got into recoil calculation when I purchased my 300 RUM. Being a handloader I didn�t feel I needed to subject myself to the 46 ft-lb�s of energy the factory 180�s develop.

With the help of a chrono and a �light� load of Retumbo my calculated recoil energy is 35 ft-lb., and it feels like 35 ft-lb as compared to my 338 at 35ish and to the 06 which hovers around 20.

The point is, I�m using an recoil energy formula that is useful for good RELATIVE comparisons; just as EPA mileage figures are OK for comparing cars.

If you haven�t noticed, barrel length is not one of the variables that influence calculated Recoil Energy. I like energy because it is harder to mitigate, unlike force which can be more easily mollified by stock design, gun hold and balance.

So, if rocket effect dependent only on barrel length is so important in the SAME firearm, will someone please post a formula on this thread that utilizes barrel length and/or gas velocity (that�s not a constant).

Ultimately, the best we can hope for is a good mathematical model for meaningful comparisons. I say we have that already being used.

By the way I got my last three shot group of Sierra 180�s to go .675 inch at 3120 fps, about 35 ft-lb in the RUM so I�m a happy camper.

Good catch!! The math shown is correct, my statement at the end was wrong. My bad....

You have to break the action of the gas into two phases, as I did, because the gas behaves very differently before and after the bullet leaves the bore. You can't treat the two situations as the same. In fact, as I think about the results we have seen, I'm very suspicious that the main contributor to felt recoil is the behavior of the gas after the bullet exits.

I believe the MD's observations about the effect of bore make excellent sense. It seems reasonable to me that a fat bore will exhaust its gas faster than a skinny bore, imparting momentum faster. Force is the time rate of change of momentum.

The 4,000 fps approximation that Scott suggested comes from the fact that we can see the result of all the momenta by observing the action of a suspended rifle as it is fired. We can easily model the first two terms, the one from the bullet and the one from the contained gas. The 4,000 fps approximation comes from knowing the typical total result, and the contributions of two of the three factors. 4,000 fps is what is needed to balance the equation. It's only an approximation, and MD's statements about bore probably modify it considerably.



I don't see how that follows from Newton's model. I think you have to break this up into the three terms that I showed. Can you explain?



You and Newton will have to duke that one out. Newton's equations say that the difference in recoil due to a loss of 100 fps of bullet speed is small. That doesn't say there is no difference in recoil. It says that the bullet alone can't account for it.

I was going to say that it isn't rocket science, but then, I guess it is.

I'll go get some actual data, and then we can have just as merry a time analyzing that. We'll know a lot more when we can see force as a function of time.

Sure, this is just the expression for the approximation of the total momentum once the bullet leaves the barrel. I'm not using any standard units here such as slug ft/sec... 286gr.* 2,400fps + 65gr*4,000fps = 1000*946.4 grft./sec. My point here is only that in many real world situations there must be a disproportionate increase in the VC of the gases over the lost velocity of the bullet leaving the barrel just to offset the lost momentum, (let alone increase it) of the bullet losing 100fps. Doesn't mean it couldn't happen but we have reason to be skeptical.
Now in the example you gave of a 180gr bullet at 2,700fps and 2,600fps w/ 60gr of powder there is a 3.7% loss of velocity from the bullet so VC the gases must increase by 7.5% just so the total momentum stays as before never mind increases.
As to the third term you are using, this is surely correct up in till the time the bullet exits. The problem I see is that 1.5 milli-sec is not a long enough time for the human body to notice, 250mS is more like it. After the bullet exits the sudden release in pressure accelerates the gases and the momentum produced overwhelms the momentum during the first 1.5 mS or so.
Lastly we must have some way of measuring recoil, we know it can't be momentum because when we increase the weight of the rifle �felt� recoil goes down. The standard formula uses recoil energy which dose go down with an increasing rifle weigh. I think your idea of using force makes more sense as long as we get the same effect of the recoil being reduced with an increase in firearm weight.

Ok... I think I understand now.

I think you've identified the nub of the problem: Losing 100 fps at typical bullet speeds has a very small effect on the recoil momentum of the rifle. So we have to look elsewhere to explain the observed result. Maybe it is just the change in weight of the rifle (don't think so, but maybe), or it's an illusion caused by blast (don't think so, but still maybe), or perhaps it is the fact that a short barrel may exhaust gas that starts at 12,000 PSI, vs. a long barrel may exhaust it at around 4,000 PSI. That sounds like the likely explanation to me.

So, as soon as my new gizmo gets here, and the snow clears, I'll trot off to the range and see if I can get us some actual data to chew on. That should be fun.

Yeah, that would make a huge increase in the velocity of the gases exiting the barrel, surely enough to overwhelm the other factors?



That would make a great science project any kid enrolled in our current educational system out here in Kalifornia <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

Well it looks like you know where I'm coming from. So, lets try this.

After the primer blast. The powder column burns and compresses the unburned powder into the shoulders and the back of the bullet (As the article said). When the bullet breaks free and starts its way down the barrel a fisher is created in the compressed unburned powder when the powder behind the bullet starts down the barrel leaving a mass of compressed unburned powder traped at the shoulders (As the article said). OK this power is now traped in a cone shape around the sholders of the round ( This is a shaped charge looking back to the primer). This will make the pressure go up and generate more heat in the case. Thus a duuble flash and a more efficient burn.

With a short fat case, you trap more powder at the shoulders and have less powder follow the bullet into the barrel. Thus less muzzle blast.

Think about this next time you slam the .25WSSM. Maybe you are old school and don't even know it.

Some more thoughts on �rocket effect" and the use of the term in ballistics:

A �true� rocket is a device that�s efficient in a vacuum or outside the earth�s atmosphere. This is because the mass being changed from solid to gas has its own mass AHEAD of it to PUSH off. It needs this DENSE mass to push off. Rocket propulsion also needs to be a steady state event to develop any real thrust. A single �explosion� or fast acting impulse of gas doesn�t amount to much. The impulse engines you may have heard about in science rags are atomic; a lot more energy! Even the awesome 30-378 Weatherby is not a good impulse rocket.

That said, a firearm powder charge is not really a rocket engine. It acts more by pushing on a projectile, not its own mass. Upon release to the surrounding atmosphere it dissipates very rapidly, unlike a true rocket stream of gas. All this also implies that bullet velocity is what�s important.

Now, don�t confuse the effectiveness of muzzle brakes with evidence of rocket effect. Muzzle brakes are always before the end of the barrel. This is an entirely different dynamic because the gas has barrel mass to push off.

Another phenomenon with gases is that the velocity is DIRECTLY proportional to the cross sectional area of confinement (barrel). When gases leave the end of the barrel what do you think happens to gas velocity? It drops like a rock. Without velocity there isn�t the momentum that rocket effect proponents try to sell. And the reason there�s no significant velocity is because a cartridge is not a �real� rocket engine.

The term �rocket-like� was also used in this thread. I maintain that this is also a stretch. Let�s just say that evidence of gas propulsion comes out the end of a barrel; rocket no. Momentum, not important. Momentum is not an expression of force or energy but mere evidence that force or energy, once upon a time, took place. <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />

gmack

Great idea! I have a nine year old grandson who loves to hunt, and who will need a science fair project next fall. It would be a blast to do that with him.

Since when does a rocket hve to have somthing to "push against"?.
At the point the bullet leaves the muzzle you have 60 or so grains of gas moving at 1/2 the bullet velocity, and at a pressure of several thousand pounds.
The gas will accelerate as it expands. It is the aceleration of the gas that cause recoil.
Good luck!

Stand with your back to a wall and move your arms like you are doing a push-up.
Now, turn around and face the wall; move your arms as if doing a push-up.

See, pushing more mass causes more recoil. Same as in a rocket. Gas from a rifle causes recoil when pushing on bullet. When it begins to push air the effect is like you doing air push-ups. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

Please, nobody tell Chuck Yeager about this. He still thinks he broke the sound barrier in a winged rocket, with nothing to push against. Or Neil Armstrong, who still thinks he took off from the moon with nothing but a rock and a vacuum within a quarter million miles.

It has (accidentally) been demonstrated that a SCUBA tank, pumped up to 3,000 PSI, and dropped neck down, so as to break the neck of the tank, results in the tank breaking a hole in the roof, and landing half a block away.

If you want your theories to stick, you're going to have to explain why gas at 3,000 PSI will propel a SCUBA tank through the roof, while 10,000 PSI gas in a gun barrel does nothing.

[quote]
At the point the bullet leaves the muzzle you have 60 or so grains of gas moving at 1/2 the bullet velocity, and at a pressure of several thousand pounds.
The gas will accelerate as it expands. It is the aceleration of the gas that cause recoil.




I'm really surprised there are no academic types in this tread. I'm certainly not one, just thought by now someone else would have backed up their statements with their qualifications.

The operative word in your quote is "leaves" the barrel. Mass that leaves a barrel with velocity only is imparting no recoil!

Pressure; I've got pressure in my truck tires. Pressure like momentum is not recoil. Pressure is unused energy so I don't care how much you got, it ain't doing much.

Gas will accelerate as it expands very well in a barrel and very poorly outside of it. Make sense. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

"Muzzle brakes are always before the end of the barrel." Wrong, they attach to the end of the barrel.Rick.