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Old November 12, 2017, 03:50 PM   #1
sixgunnin
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(off topic) Recoil To Bullet Speed?

I have been thinking about long range shooting and got to wondering. Could how solidly you hold a rifle effect the bullet speed and drop? How much higher velocity's could be achieved firing with the butt of a rifle against something solid vs holding it loosely? Would it make much difference? Or any?
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Old November 12, 2017, 04:11 PM   #2
RIDE-RED 350r
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I am no expert ballistician, but I would say the difference would be at most negligible.
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Old November 12, 2017, 04:39 PM   #3
hdwhit
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Quote:
sixgunnin wrote:
Could how solidly you hold a rifle effect the bullet speed and drop?
Yes.

Quote:
How much higher velocity's could be achieved firing with the butt of a rifle against something solid vs holding it loosely?
Assume your "loose" hold allows the gun to recoil at 5 feet per second (fps) and that holding it against something solid reduced that to zero, you would pick up 5 fps in velocity.

Since 5 fps is within the likely standard deviation of the ammunition (either factory or hand loaded), the overall effect on the shooter would likely not be noticeable.
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Old November 12, 2017, 04:53 PM   #4
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Quote:
Assume your "loose" hold allows the gun to recoil at 5 feet per second (fps) and that holding it against something solid reduced that to zero, you would pick up 5 fps in velocity.
I am not aware of any empirical studies on this point -- or any detailed analyses -- for the simple reason that it is likely of no practical utility. However, I believe the quoted response is likely not correct.

Muzzle velocity of a rifle bullet is fixed at the moment the slug exits the muzzle (at which time it is no longer influenced by combustion chamber pressure).

Recoil of the rifle, on the other hand, almost entirely occurs after the bullet has exited the muzzle. There is an engineering explanation for this, but there are also a host of high speed video stills (and, before that, stroboscopic photographs) illustrating that the muzzle does not begin to move in recoil until the slug has exited.

Since the effect of the hypothesized fixing or relaxing of a buttplate restraint only occurs after muzzle exit, and thus after bullet muzzle velocity is fixed, it can have no effect on muzzle velocity.
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Old November 12, 2017, 05:01 PM   #5
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No appreciable difference.
If you go look at YouTube for slomo of bullets exiting high powered rifles, the bullet is out of the barrel before there is any appreciable gun recoil.

Pause the Youtube video and use the slider bar to see the bullet leave the barrel followed by the recoil.

.50 BMG Recoil occurs after tracer round leaves the barrel


This is a small segment of the Video, and is copyrighted and used with permission.
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Old November 12, 2017, 05:56 PM   #6
dahermit
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If you go look at YouTube for slomo of bullets exiting high powered rifles, the bullet is out of the barrel before there is any appreciable gun recoil.
Then there is no practical purpose in using the same "spot weld" on the stock as taught in Basic Training?
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Old November 12, 2017, 06:12 PM   #7
Art Eatman
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Accuracy is a function of proper hold of a rifle. A snug, firm mounting reduces felt recoil and makes consistency more doable.

Nothing you do on the outside of a rifle will affect the muzzle velocity. MV is strictly a function of the powder and primer.
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Old November 12, 2017, 07:57 PM   #8
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There was a guy who used to post here regularly who claimed to have seen data proving that a rifle held solidly did gain some velocity over one that recoiled freely. I haven't seen anything from him lately. IIRC he claimed differences as much as 50-100 fps.

I can believe there could well be a difference, but that number seems high to me. But even if true, 50fps would not make a huge difference in trajectory until you get WAAAAY past where most of us shoot.
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Old November 12, 2017, 08:34 PM   #9
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Accuracy is a function of proper hold of a rifle. A snug, firm mounting reduces felt recoil and makes consistency more doable.
I would have to disagree with the snug, firm part. I have switched to free recoiling my 1k bench rifle and have considerably closed the groups. Get the rifle lined up perfect, get off It, tap a 1.4oz trigger. Just remember to baby powder the stock and bags between relays.

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Old November 12, 2017, 09:12 PM   #10
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^^^ This

I shoot much smaller groups with very light control of my .50 cal.
At 1000 yards, you can see your heartbeat in the scope of you grip it tightly.
A light grip reduces another variable. (Heartbeat)
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Old November 12, 2017, 10:02 PM   #11
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"Then there is no practical purpose in using the same "spot weld" on the stock as taught in Basic Training?"

Consistency in holding the rifle has an positive affect on accuracy but that wasn't the question of the OP. OP question was about velocity not accuracy.
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Old November 13, 2017, 07:53 AM   #12
Art Eatman
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TXAZ, learn to do as the Olympic shooters: Shoot between heartbeats. After all, you have almost a full second.
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Old November 13, 2017, 10:40 AM   #13
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Originally Posted by Art Eatman View Post
TXAZ, learn to do as the Olympic shooters: Shoot between heartbeats. After all, you have almost a full second.
Have thought about that Art, but that isn't my physiology unfortunately.
But my Barrett sits on a bipod and monopod so that helps some.
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Old November 13, 2017, 12:47 PM   #14
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Quote:
Originally Posted by RKG
Recoil of the rifle, on the other hand, almost entirely occurs after the bullet has exited the muzzle. There is an engineering explanation for this, but there are also a host of high speed video stills (and, before that, stroboscopic photographs) illustrating that the muzzle does not begin to move in recoil until the slug has exited.
Actually, none of that is correct. If recoil had no effect on muzzle position before the bullet cleared the muzzle, different load levels would not cause the elevation in point of impact (POI) to change by more than the difference in drop due to difference in velocity, which is considerably smaller than the change an Auddette ladder reveals. The change during the millisecond it takes the bullet to leave the muzzle is not much, and you usually have to put a stationary line on the slow motion movie to distinguish it, but it happens. If it didn't, barrel tuners would have no effect. (I note some slow motion images are from guns in machine rests, so these may exhibit no visible movement.)

Recoil has two stages. In a rifle, the first stage occurs while the mass of the bullet and about half the mass of the powder charge get pushed down the bore. The second stage is variously called "rocket effect" or "after-effect". It occurs when the bullet clears the muzzle, uncorking the pressure behind it, which then accelerates the mass of the gas inside forward at even higher velocity, causing additional push back. In some overbore cartridges, this can account for half the total recoil energy given to the gun. If you ever wondered why a muzzle brake that vents gas equally in all directions, pushing up as much as it pushes down, can still tame part of the recoil, this is the reason: it vents the gas sideways instead of forward, leaving little high pressure gas available to produce rearward rocket effect. It does nothing about the portion of the recoil due to the bullet mass going forward, so there is still recoil.

What drives the gun back is Newton's third law of motion. It is most commonly shortened to state: for every action there is an equal and opposite reaction. What this means is simply that the momentum of all the stuff pushed out of the muzzle, bullet, gas, unburned powder, etc.; aka, the ejecta) will be equal and opposite in direction to the momentum imparted to the gun and, through it, to the shooter, and through the shooter, to the earth. You cannot move the bullet down the bore without picking up momentum, due to the equal and opposite force applied by pressure to the bullet and to the breech of the gun. It's inescapable.

Fortunately for the shooter, while momentum is equal and opposite, energy is not. This is what keeps the gun from damaging the shooter as much as it does the target. Yes, yes, the bullet is pointy and contacts less area, but when you look at the vast permanent cavity of damage caused by a high power rifle bullet that expanded or tumbled, you realize the diameter of that damage is often even bigger than the butt of a rifle, yet the shooter didn't experience such damage.

SAAMI has a free document describing how to calculate gun recoil energy (it is mainly what you perceive as recoil).

As to the velocity of the recoiling gun, that's very easy. Because momentum is equal and opposite and created simultaneously, if the rifle were free floating and if you have a perfect muzzle brake to take all the rocket effect out, the rearward speed of the rifle would just be the speed of the bullet divided by how many times heavier gun is than the weight of the bullet plus about half the powder charge. If the gun weighs 8 lbs and fires a 150 grain bullet with a 40 grain charge of powder to 2800 fps, then 8 lbs is 56,000 grains, and 56,000 grains divided by 150+20 grains is 329.4, and if you divide 2800 fps by 329.4, you get 8.5 ft/s. Add in rocket effect for a .308 W and you might be in the range of 11 fps. During the acceleration of the bullet the average velocity of the gun will be half that, or 5.5 fps, so in the millisecond it takes the bullet to clear the muzzle, the gun will have moved 0.0055 feet, or 0.066 inches or a little over a 1/16th of an inch. And this assumes you are making no contact with it, not loading it with your own body mass (which adds to that 8 lbs). It's not much.

To the OP's question: The effect of this recoiling movement does not change the pressure on the bullet or on the gun breech, so the gun's velocity is not subtracted directly from the bullet's, as it would be if the gun were moving backward at the start of the shot. That would be subtracted because the backward velocity would already be in the bullet. But in the recoiling gun, the gun doesn't move until the bullet does. So the bullet never has rearward velocity to subtract. At that point you are just looking at two independently moving bodies with the same pressure acting on both. Instead, the recoil velocity effect will be that of leaving the bullet with 0.066 fewer inches of barrel length by the time it reaches the position of the muzzle. For the 150 grain .308 W I described, it amounts to about 1.5 fps velocity loss.

I think most of the difference in velocity claimed for different shooting positions has to do with a combination of powder position in the cartridge being changed and with firing angle over the chronograph being changed and introducing a measuring error. The theoretical difference for the free recoiling gun is just too small, and one pressed against a body will lose even less.

As to energy, that is equal to the work done on the bullet. It is the distance of travel in the gun times the average (not the peak) force in pounds applied to the base of the bullet by pressure. It even has the same units as energy: ft-lbs. So if the bullet has 2611 ft-lbs at the muzzle and the barrel was 23.934 inches long (after subtracting 0.066 inches), you had and average force of 1305.5 lbs at its base during its whole time in the barrel. That ignores the equivalent of half of the powder mass traveling with it. That would have gained about 12% of the energy of the bullet, so the total force applied to both was actually about 1480 lbs, and that's what the rifle will see. Applied to the recoil distance of the gun, you have 0.0055 ft time 1480 lbs force, or about 8.14 ft-lbs of energy in the gun. That's ideal. In closer approximation, QuickLOAD identifies both unburned powder mass and half the burned mass moving forward and comes out with more like 9.24 ft-lbs. The rocket effect raises that to almost 15 ft-lbs. But the bullet has 2611 ft-lbs. No comparison, and the shooter feels that much less.
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Old November 13, 2017, 02:15 PM   #15
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Unclenick sez:
Quote:
I think most of the difference in velocity claimed for different shooting positions has to do with a combination of powder position in the cartridge being changed and with firing angle over the chronograph being changed and introducing a measuring error. The theoretical difference for the free recoiling gun is just too small, and one pressed against a body will lose even less.
I did this science experiment a few years ago using my 308 bolt gun. I loaded fifty rounds identically. I sized and trimmed all of the brass identical and loaded 44 grains of AA 2495 under a Sierra Match King 150 gr bullet. I fired 25 rounds using an anchored weighted lead sled and 25 rounds using my Wichita rest allowing normal recoil into my shoulder. The chronograph used was an Oehler 35P.

Lead Sled Setup:


Wichita Rest Setup:


The Results:


Using twenty five shot samples there was no real discernible difference in velocity.

In addition to Unclenick's link to the SAAMI formula Hatcher's Notebook also has a section covering free recoil energy.

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Old November 13, 2017, 02:25 PM   #16
T. O'Heir
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"...how solidly you hold a rifle effect..." No. How you hold the rifle affects accuracy and nothing else.
Bullet speed(MV) is about the amount of powder gases driving it, the mass of the bullet and length of the barrel.
Bullet drop is about velocity loss and gravity with a wee bit of air resistance thrown in. None of which you can affect.
"...bullet is out of the barrel before there is any appreciable gun recoil..." Recoil starts at exactly the time the bullet starts moving in the cartridge. It's called physics. You just don't notice it until the bullet has left the barrel.
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Old November 13, 2017, 03:42 PM   #17
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No need to elaborate....but

Putting the butt of your rifle against a tree...etc is a good way to fracture your stock. The manufacturer would likely deny the warranty claim as well.

A lot of the big boomers have an Addl. cross pin in the stock to reinforce it.

I would not risk damaging the rifle, and as we have seen here, it really makes no difference in velocity.
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