[cowboy1964]

I've seen several people make the claim that heavier bullets make the POI higher because the bullet won't be out of the barrel before the muzzle starts rising. I find that difficult to believe but have no proof one way or the other. Can anyone prove this?

[vafish]

Well for the bullet to start moving down the barrel some force has to be applied to the bullet. For every force there is an equal and opposite force, meaning there has to be recoil going back to the gun as the bullet is moving down the barrel. As the gun recoils the muzzle rises.

It seems more prevalent in handguns and at closer ranges.

Here is a high speed video of a .22 rifle being fired. You can clearly see the barrel moving back before the projectile exits.

http://s109.photobucket.com/albums/n...t=22_rifle.flv

[481]

Originally Posted by cowboy1964

I've seen several people make the claim that heavier bullets make the POI higher because the bullet won't be out of the barrel before the muzzle starts rising. I find that difficult to believe but have no proof one way or the other. Can anyone prove this?

vafish has it right.

For a mass to be accelerated it must be subjected to a force, F = ma.

That action applies to both bodies involved, the gun and the projectile (see Newton's Third Law mv = mv): "To every action an equal and opposite reaction."

This effect is instantaneous for both bodies, the projectile being set into motion by the force applied and the gun being acted upon by the same accelerative force, so the barrel is in motion before the projectile, no matter its weight, exits. The bullet is therefore in the barrel as it starts to rise.

The only way that a bullet could ever leave the barrel before it begins to rise is that it would need to achieve an instantaneously infinite velocity at primer ignition meaning that it woud take zero time to transit the barrel's length and the barrel would not have a chance to rise. (that would be quite a handload. )

Newtons Laws of Motion are the proof for these phenomena.

[Gunnut 45/454]

The simple answer is yes! Remember newtons law! Theres a equal reaction to the force required to move a mass! As you saw from the lowly 22LR! Multiply that 10x to move a 230 gr 45acp bullet down a shorter lighter firearm! The slide is also a major factor in muzzle jump along with the design of the hand grip!

[Berto]

You can also watch videos of the 1911 in action, where the barrel is about to unlock from the slide just as the bullets exits the muzzle. Dialing in fixed sights on a short barreled handgun involves quite a bit of 'science'.

[DonD]

Anyone with significant experience with really big bore handguns can tell you that what the previous posters have said is true in the real world not just in theory.

There is a huge, dramatic difference in POI with a .500 Mag between say a 440 gr slug at 900fps and one at 1600 fps. Even at ranges as close as 25 yds, the fast load may print 8" lower than the slow one. If you want to use both loads, you have to have two different elevation settings on your rear sight. Don

[wanderinwalker]

Originally Posted by DonD

Anyone with significant experience with really big bore handguns can tell you that what the previous posters have said is true in the real world not just in theory.

There is a huge, dramatic difference in POI with a .500 Mag between say a 440 gr slug at 900fps and one at 1600 fps. Even at ranges as close as 25 yds, the fast load may print 8" lower than the slow one. If you want to use both loads, you have to have two different elevation settings on your rear sight. Don

I don't think you need to get even that big. A .44 will show a major POI/POA shift at 25 yards going from 200gr @ 900 to 240gr @ 1300.

Now, what I am trying to wrap my mind around is why does my 4" .357 print 4" high at 25 yards with a 158gr JHP at 1225-fps versus a 158gr .38 SWC chugging along at 800? My theory is that it is the firmness of the grip and the recoil force; my high grip keeps the muzzle down more with the lighter load, while the heavier load can generate enough force to move the pistol more, faster, causing the muzzle-arc to be higher at exit.

[DonD]

Originally Posted by wanderinwalker

I don't think you need to get even that big. A .44 will show a major POI/POA shift at 25 yards going from 200gr @ 900 to 240gr @ 1300.

Now, what I am trying to wrap my mind around is why does my 4" .357 print 4" high at 25 yards with a 158gr JHP at 1225-fps versus a 158gr .38 SWC chugging along at 800? My theory is that it is the firmness of the grip and the recoil force; my high grip keeps the muzzle down more with the lighter load, while the heavier load can generate enough force to move the pistol more, faster, causing the muzzle-arc to be higher at exit.

I agree with you on point one. I don't know what to say on the second paragraph, your theory seems reasonable to me. There can always be an outlyer to screw up the best data! Don

[English]

Originally Posted by cowboy1964

I've seen several people make the claim that heavier bullets make the POI higher because the bullet won't be out of the barrel before the muzzle starts rising. I find that difficult to believe but have no proof one way or the other. Can anyone prove this?

I think the posts above have already done the proving but the question is interesting. Muzzle flip is a consequence of the recoil impulse being out of line with the resistance. The resistance consists of the inertia of the gun and the shooter's grip on the gun.

So with a Glock where the very large majority of the mass of the pistol, the barrel and slide, is very close to the barrel axis, the inertial component of muzzle flip is low. That is, if you could fire it without gripping it, it would flip less than if you had it fitted with a solid metal plug in the butt.

Then, the greater the perpendicular distance between the barrel axis and the center of presure of your hand on the backstrap (and the less the lever length and strength of your grip on the bottom end of the front strap) the greater the muzzle flip.

Both of these things happen instantaneously as the bullet accelerates up the barrel and so the muzzle flip starts to happen as soon as the bullet starts moving. This affects revolvers much more than auto pistols. With a revolver, the full effect of accelerating the frame of the pistol backwards has happened by the time the bullet has just left the barrel. With an auto pistol the greater part of that momentum transfer is still located in the slide and so the frame "feels" only small compression of the recoil spring by the time the bullet has just left the barrel. The shooter gets the same muzzle flip, other things being equal, but almost all of the acceleration to the frame, and therefore the point of aim, is delayed till after the bullet has left the barrel.

The interesting consequence, which I had not actually realised till I started to think about your question, is that the point of aim of auto pistols is much less sensitive to bullet momentum than is the case for revolvers. I have understood for a long time how it is that felt recoil is reduced in autos relative to revolvers of the same weight but the reduction of POA sensitivity is a new thought for me. Thank you for that - I keep looking for new thoughts!

English

[English]

Originally Posted by wanderinwalker

...
Now, what I am trying to wrap my mind around is why does my 4" .357 print 4" high at 25 yards with a 158gr JHP at 1225-fps versus a 158gr .38 SWC chugging along at 800? My theory is that it is the firmness of the grip and the recoil force; my high grip keeps the muzzle down more with the lighter load, while the heavier load can generate enough force to move the pistol more, faster, causing the muzzle-arc to be higher at exit.

It isn't bullet weight that matters but bullet momentum. The 357 has slightly more than another 50% recoil and so it flips more. The key concept is that both flip before the bullet leaves the barrel.

English

[wanderinwalker]

Originally Posted by English

It isn't bullet weight that matters but bullet momentum. The 357 has slightly more than another 50% recoil and so it flips more. The key concept is that both flip before the bullet leaves the barrel.

English

Sounds like we're on the same train of thought here. It just runs counter to the theory of a higher velocity load shooting lower because it gets out of the barrel faster. But as you wrote above, there is much more momentum involved when going from .38 Special to .357, especially compared to shooting 115gr 9mm versus a 147gr 9mm.

I think you're on to something about revolvers being more sensitive to POI/POA shifts due to varying ammunition. Let's also not forget that most autopistol cartridges operate in a very narrow power range, as opposed to the rather large difference between .38 Special LRN ammo and .357 full-power hollowpoints. In this light, the difference between 200gr .45 ACP and 230gr .45 ACP is minor.

[Gunnut 45/454]

wanderinwalker
If you lock a pistol in a fixture you do not see as much difference in POI! The difference you see is the effect of the recoil impulse in the shooters arm/hand! The grip configuration of the pistol is key- as been said glock recoil axsis is pretty much in a straight line with the shooters arm-minimizing the effect of recoil/muzzle flip! Look at all the "RACEGUNS" used in compitition they all have very straight aliment with the shooter arm! Same goes with rifles Compitition /sniper rifles are straight line impluse with recoil!

[wanderinwalker]

Originally Posted by Gunnut 45/454

wanderinwalker
If you lock a pistol in a fixture you do not see as much difference in POI! The difference you see is the effect of the recoil impulse in the shooters arm/hand! The grip configuration of the pistol is key- as been said glock recoil axsis is pretty much in a straight line with the shooters arm-minimizing the effect of recoil/muzzle flip! Look at all the "RACEGUNS" used in compitition they all have very straight aliment with the shooter arm! Same goes with rifles Compitition /sniper rifles are straight line impluse with recoil!

Like my AR-15 Match Rifle? (Well, actually, it's a Service Rifle, but it IS an accurized AR.)

Yes, I do understand that a pistol or rifle fired from a fixed fixture will show little to no POI shift until you reach the ranges where trajectory differences in bullets/loads become more pronounced. Like at 600-yards with my AR, a 77gr Nosler HPBT needs an extra two and a half minutes (15" of drop difference) of elevation versus an 80gr load. But at 100-yards, the difference is negligable from a 52 to the 80s (no more than a couple of inches).

And at any rate, the firearm is beginning to recoil before the bullet is out the barrel!

[Eagle22]

These might help:

Right hand shooter diagnostic chart
http://www.apdsnc.com/Brochure/CorrectionChartRight.pdf

Left hand shooter diagnostic chart
http://www.apdsnc.com/Brochure/CorrectionChartLeft.pdf

recoil can be reduced by how you hold your gun.

When you look at these charts, you will see that shot placement was effected by barrel movement, which is grip related.

[-gunut-]

I have read that the sights on S&W J-frame revolvers are set for FBI 158gr ammo, so using smaller gr. bullets will give you different POI.