Physics question: recoil vs. POI ?

D

dyl

New member
**Update: verdict is: recoil starts as soon as the projectile starts moving in the barrel. A consistent grip affects firearm movement during recoil impulse, which affects POI.

Alright you physics nuts, here's a mind bender for you.

Premise 1: I have heard that in semi auto pistols, recoil/barrel movement/slide reciprocation does not occur until after the projectile exits the barrel. All forces in the chamber/barrel are balanced until the projectile exits, then the barrel acts like a rocket pointed towards the breech face and starts the cycling motion.

Premise 2: The above does not hold true for revolvers. J-frame snub nose manuals state that sights are calibrated for 158 grain projectiles. Lighter projectiles impact lower. This has been explained as longer dwell time in the barrel AS THE BARREL RISES FROM RECOIL (even before bullet exited the barrel). Lighter bullets will exit earlier on the upwards ark and impact lower.

Question: If the above are true, why the difference? Why does recoil happen in a revolver before the bullet has exited the barrel but not on a semi auto?

My guess: perhaps barrel cylinder gap allows gasses to escape and pushes the revolver up even before the bullet exits the barrel.

Please help me clear this mind twister up.
 
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Premise 1: I have heard that in semi auto pistols, recoil/barrel movement/slide reciprocation does not occur until after the projectile exits the barrel.
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Not true. Look for some high-speed video of auto pistols cycling; the slide and barrel are moving before the bullet exits.
Not only that, the gun is moving in the shooter's hand while the bullet is in the barrel. That's why the barrel is pointed downward when the sights are aligned on target, to counter the muzzle rise while the bullet is still in the barrel.
Also, grip strength will affect how much the gun recoils, and that's why you can't have someone else sight-in your gun for you, as the bullets won't hit the exact same point when the gun is held by two different people, or when one person does not use a consistent grip.

Definitely not a physics nut, though I did take a class called Physics for Liberal Arts Majors, and have stayed in a Holiday Inn.
 
Premise 1: I have heard that in semi auto pistols, recoil/barrel movement/slide reciprocation does not occur until after the projectile exits the barrel. All forces in the chamber/barrel are balanced until the projectile exits, then the barrel acts like a rocket pointed towards the breech face and starts the cycling motion.
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Yep that's incorrect recoil impulse starts as soon as bullet moves.

now on a locked breach auto IE 1911 the barrel won't come unlocked until after bullet is gone.
 
Posted by mavracer:
recoil impulse starts as soon as bullet moves.
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Bingo!

As the bullet and the propellant gases move in the barrel, parts of the gun and the shooter's hand and arm move in the opposite direction.

now on a locked breach auto IE 1911 the barrel won't come unlocked until after bullet is gone
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Right.

On a 'blowback' pistol, the unlocked slide starts moving immediately as the bullet moves forward.

The reason for the "muzzle rise" that RickB mentions is that because the bore axis is above the shooter's hand, and as the barrel moves backward, the pistol pivots around the fulcrum formed by the shooter's hand and wrist. I'm sure there is a better way to express that.
 
And that's why some are rightly concerned about low bore axis and gripping the gun as high on the frame as possible, to reduce the length of the "lever" of the gun acting on the shooter's grip.
 
dyl said:
Lighter projectiles impact lower. This has been explained as longer dwell time in the barrel AS THE BARREL RISES FROM RECOIL (even before bullet exited the barrel). Lighter bullets will exit earlier on the upwards ark and impact lower.
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I've never been a believer that it's dependent on velocity (i.e. dwell time).

Solving the equation for conservation of momentum between the gun & bullet while the bullet's still in the barrel shows that the "time" element cancels out, and that muzzle rise (while the bullet's still in the barrel) is dependent solely on the masses of the bullet & gun, and the length of the barrel. Not time (and therefore not velocity).

All else equal, lighter bullets are faster, so it appears as though it's velocity-dependent. But while the faster bullet may spend less time in the barrel, the effect is canceled out because it also spends correspondingly less time pushing the gun.
 
It's called relativity. Increased recoil and dwell time means increased change in POI. The bigger the difference in recoil and barrel dwell, the bigger the difference in POI. For example....difference between a lightweight 185 gr .45ACP and a heavy weight 230 grainer is only 45 grains and difference in velocity is only about 100 fps. Small difference relates to a small difference in POI. In a .44 mag, difference in bullet weight can be as much as 120 grs and a difference in velocity of 500 fps or more. Much bigger difference thus a bigger difference in POI. Add to the fact most .45 ACPs are shot at distances of 25 yards or less, while most .44s are shot are ranges of 25 yards or more. Longer distance means bigger differences in POI. It' not rocket science and very little to do with the platform they are shot from.
 
dyl,

The POI for auto pistols is effected by the interaction of the recoil and grip.

The best example I can offer is: Marine showed up at the gun range with a new desert eagle in 50AE. He offered me to shoot it with him. At 15 yards our groups were 15" apart when shooting to the same POA. This difference was not a result of sighting difference at that close distance.
 
buck460XVR said:
Longer distance means bigger differences in POI.
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Trajectory is an "external ballistics" issue. POI changes with regard to bullet weight is about "internal ballistics". POI changes due to bullet weight pushing on the gun has to be considered at distances that're close enough that trajectory differences aren't significant.

When shot at, say 15 - 20 yards, I've never noted any POI difference between a full-house 158gr .357mag and a powder-puff 158gr .38spl load. I'll have to check again on my next visit to the range.
 
In my experience one can vary the vertical impact point by shooting heavier or lighter bullets in handguns, with the heavier bullets striking higher on the target. The effect is more noticeable with slower moving, heavier, projectiles. I notice the effect more in a 45 ACP than I do with 38 specials. Too I notice that lighter powder charges tend to shoot higher than heavier charges with the same bullet.

Sometimes the effect can be mitigated with a higher hold on the handgun, if possible.

I do not understand the physics all that well, but I do understand the effect on the target. :) I liken my experience to the "butt dyno" readings on the acceleration of different motorcycles. Some feel faster than others. Highly unscientific, but valid for my purposes.
 
There's so many things at work here, that what causes changes in the point of impact in one gun, with one shooter, may not hold for another gun and shooter combination.
Maybe that's why there seems to be so many explanations that can contradict one another.
To correct for discrepancies in point of aim and point of impact, each shooter should just make the corrections in the sights or hold to compensate as necessary.
Like the neighbor physicist and ballistic expert told me,
"After employing the theory and noodle work, to verify just shoot 'em and see where they land."
 
I see

Yes, I love this kind of info. Thanks to each of you. You are one reason why this forum is so good.

My mind is blown. And here I thought a consistent grip/stance was to normalize our *interface* with the pistol with regards to throwing trajectory off with poor trigger control or other muscular inconsistency.

So it turns out our grip orientation and grip pressure affects trajectory because it also affects the firearm's actual motion during the recoil impulse *before* the bullet exits the barrel. So this is yet another reason why ransom rests are effective - consistent interaction with the recoil impulse. Maybe not noticeably at 7 yards but it's good to know what's going on :)

It should be noted that I first thought recoil doesn't start until the projectile leaves the barrel - because someone told me: look at high speed footage, recoil doesn't happen until the bullet leaves. I'm guessing they meant the slide didn't move but it was still unlocking in reality.

I had understood the importance of this on rifles as our hold on the stock tweaks the stock and puts pressure on different areas of the stock.

MrBorland - I don't think I followed your post well. My previous thinking was that given the same powder charge, lighter projectiles exit the barrel "sooner" on the uprise of the barrel causing a lower POI. I had forgotten that a lighter projectile would also cause less recoil impulse/muzzle rise. Were you saying that the lower POI is actually due to less muzzle rise rather than less time in the barrel?
 
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dyl said:
Were you saying that the lower POI is actually due to less muzzle rise rather than less time in the barrel?
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Yes.

It's momentum (mass x velocity) of the bullet pushing against the movable gun while it's still in the barrel. For a constant barrel length, relative momentum becomes the product of mass and time.

Take time first: A faster bullet has exactly that much less time to push the muzzle up while it's still in the barrel. Ergo, fast or slow doesn't matter - it'll push the muzzle up the same distance while traveling down the barrel, all else being equal.

Now consider mass: If the mass of the gun doesn't change, the heavier bullet gains a "push advantage" against the gun, so bullet mass doesn't cancel out, and the heavier bullet will push the muzzle up more while it's traveling down the barrel.
 
I first thought recoil doesn't start until the projectile leaves the barrel - because someone told me
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This misconception seems to be out there and I'm resigned to the fact that it's never going to die.

(And the U.S. Army went with the M16 because they want to WOUND the enemy-not kill them-because the WOUNDED soldier would require the attention of at least two other enemy...) sigh. Even Snopes can't kill these things.
 
Premise #1 is incorrect.

Recoil impulse begins as soon as the bullet begins moving. The gas pushing the bullet is pushing the case (and the gun) too.

FULL recoil energy doesn't happen until the bullet leaves the muzzle.

Movement of movable parts begins as soon as there is sufficient recoil energy to overcome their inertia, and spring pressure (if any).

Unlocking of parts (in locked breech designs) happens at a specific point in the movement cycle. Actual pressure at the time is irrelevant to the mechanical unlocking. When the parts physically move to a certain point, they unlock.

Your grip on the gun can have an effect on its POI, but can only affect that portion of recoil movement which happens before the bullet leaves the barrel.

As to whether it is time of barrel transit, or momentum over time, or something else, I don't know. I think it is the combination of things, and have no idea which one is, if one is, the dominant factor. Does the heavy bullet move the barrel higher because takes longer or because it pushes harder?

I think you could find out for yourself with a little handloading. Try this, load the light bullet to the same speed as the heavy one. then see which one strikes lower.
 
If you have something like below and reload it is obvious. This TC has a .22 barrel on it but I have 357 and 44 Mag barrels for it also. Even if you don't reload you could go from 38 Special to 357 Mag or 44 Special to 44 Mag and see the results for yourself. For every reaction there is an equal and opposite reaction which starts when the primer ignites the powder and the bullet starts moving forward and the gun starts moving rearward.

 
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