heavy bullet versus light bullet point of impact

[rintinglen]

260626
From time to time, questions have arisen concerning the effect of bullet weight on point of impact, principally in relationship to to fixed-sight revolvers. Generally speaking, slow, heavy bullets will print higher than lighter, faster ones, at least in handguns. But why, you may ask, is this so?

The reason for this is due to two factors: dwell time, the length of time a projectile is passing through the barrel, and recoil, the force exerted rearwards by the expanding powders when the propellant explodes (or, more accurately in the case of smokeless powder, combusts.) How these two elements interact determine where the bullet will impact the target.

In the above drawing we see the force of recoil designated as the arrowed line AB. This force pushes against the recoil shield causing the revolver to pivot like a lever on the fulcrum at approximately Point C. In turn, this causes the muzzle to ride upward on a curve similar to AD. Where the bullet exits the barrel along that curve determines how high on the target the bullet will strike.

For example, let us consider two 38 Special standard pressure loads. First, a 158 grain lead bullet that travels at 850 FPS, and second, a 110 grain JHP moving at 985 FPS. We have for the sake of argument, a 6 inch barrel, (including chamber throat, barrel-cylinder gap, forcing cone etc.) The 158 grain bullet will traverse the barrel in ~.0006 seconds, while the 110 grain bullet stepping out more swiftly will be out of the barrel in ~.0005 seconds. Assuming all else remains the same, (gun, grip, sight picture, pressure,) the heavier bullet will rise further in that .0001 seconds than the lighter one does. Not much , you might think, and you'd be right. But wait, there's more!

Consider also that the recoil of the 158 grain bullet is also proportionately greater than that of the 110 grain bullet. The curve, AD, for the 158 grain bullet will be longer, for the force AB is greater. So not only does it push longer, it pushes harder, causing the muzzle to rise further.

So in a nut shell, faster hits lower than slower, heavier hits higher than lighter, at least assuming similar pressures. So the next time you are trying to get a fixed-sight handgun to shoot where you point it, you should consider what you want to do, higher or lower, and adjust the load accordingly.

[tazman]

Very true.
I found a load for my 38 special with a 125 grain boolit that shoots to the same point of impact as my full power 158 grain boolits. By comparison, the 125 is creepy crawling along. It is probably slower than my wadcutter loads but since I haven't chronographed it, I can't say for sure.
I know it is slow and has very low recoil but it is undoubtedly spending some extra time in the barrel in order to shoot where it does.

[35 Whelen]

260626
From time to time, questions have arisen concerning the effect of bullet weight on point of impact, principally in relationship to to fixed-sight revolvers. Generally speaking, slow, heavy bullets will print higher than lighter, faster ones, at least in handguns. But why, you may ask, is this so?

The reason for this is due to two factors: dwell time, the length of time a projectile is passing through the barrel, and recoil, the force exerted rearwards by the expanding powders when the propellant explodes (or, more accurately in the case of smokeless powder, combusts.) How these two elements interact determine where the bullet will impact the target.

In the above drawing we see the force of recoil designated as the arrowed line AB. This force pushes against the recoil shield causing the revolver to pivot like a lever on the fulcrum at approximately Point C. In turn, this causes the muzzle to ride upward on a curve similar to AD. Where the bullet exits the barrel along that curve determines how high on the target the bullet will strike.

For example, let us consider two 38 Special standard pressure loads. First, a 158 grain lead bullet that travels at 850 FPS, and second, a 110 grain JHP moving at 985 FPS. We have for the sake of argument, a 6 inch barrel, (including chamber throat, barrel-cylinder gap, forcing cone etc.) The 158 grain bullet will traverse the barrel in ~.0006 seconds, while the 110 grain bullet stepping out more swiftly will be out of the barrel in ~.0005 seconds. Assuming all else remains the same, (gun, grip, sight picture, pressure,) the heavier bullet will rise further in that .0001 seconds than the lighter one does. Not much , you might think, and you'd be right. But wait, there's more!

Consider also that the recoil of the 158 grain bullet is also proportionately greater than that of the 110 grain bullet. The curve, AD, for the 158 grain bullet will be longer, for the force AB is greater. So not only does it push longer, it pushes harder, causing the muzzle to rise further.

So in a nut shell, faster hits lower than slower, heavier hits higher than lighter, at least assuming similar pressures. So the next time you are trying to get a fixed-sight handgun to shoot where you point it, you should consider what you want to do, higher or lower, and adjust the load accordingly.

Good write-up, but I'm not sure about the dwell time thing, even though it is oft quoted. I think it depends...

If we load in our .38 Special a 110 gr. bullet, and a 200 gr. bullet, both at 700 fps, they both spend the same amount of time in the barrel, but we know the 200 gr. bullet will shoot higher because of muzzle rise due to heavier recoil.

That said, if the bullets weights are equal or at least similar, I agree with the dwell time thing. My normal load for my .357's is a 155 gr. SWC at a little over 1,000 fps. A few days ago I was testing some similar bullets only MV was a little over 1300 fps and at 75 yds. they definitely shot lower than the slower load.

35W

[Silver Jack Hammer]

Heavier boolits impact higher than lighter boolits. Slower moving boolits hit higher than faster boolits do.

True that. The OP on dwell time is based on recoil arc.

Recoil arc should also be affected by the height of the axis of the bore. The higher the axis of the bore the greater the recoil arc.

If all the ranges were not closed right now I’d try a 230 gr. boolit at 830 fps out of a Colt’s SAA, and the same boolit weight and velocity out of a 1911. Then compare a 200 gr. boolit at 1,000 fps out of a Colt’s SAA and the same boolit weight and velocity out of a 1911.

The difference in the point of impact should be greater out of the SAA with its higher axis of the bore.

[BigAlofPa.]

I have a ruger black hawk 40sw and 10mm. I have to change my aim when going from 10mm to 40sw. It drove me nuts at 1st trying to zero it in.

[Win94ae]

The revolver is not a closed system the entire time the bullet is in the bore; the 1911 is.


https://www.youtube.com/watch?v=5XAxZc41RzQ

Once the bullet leaves the chamber of a revolver, then the barrel and bullet is in an open system; which has external interactions. This is why you don't see this in manual loading firearms or firearms that cycle only after the bullet leaves the bore.

But it can be very confusing as you can see in this video.


https://www.youtube.com/watch?v=wpO2sPqCFMw

So the theory is not quite set in stone.

[rintinglen]

In order for the bullets to be traveling at the same speed, the heavier has to be loaded at a higher pressure and hence recoils more, during the same time frame. It is physics, not theory, and if you spend half a day calculating free-recoil energy, measuring muzzle rise and accurately determining velocities, ballistic coefficients atmospheric pressure and a few other varibles that I don't recall off the top of my head, you can precisely calculate bullet estimated trajectory. Of course, when you start shooting, you will very likely discover a variance between the observed value and the expected value, which you could the use to determine the standard deviation and then calculate an appropriate chi-square value to see if your results were within an acceptable confidence level.

But I'd rather go shooting, and then adjust my loads empirically to get the result I want.

[tazman]

In order for the bullets to be traveling at the same speed, the heavier has to be loaded at a higher pressure and hence recoils more, during the same time frame. It is physics, not theory, and if you spend half a day calculating free-recoil energy, measuring muzzle rise and accurately determining velocities, ballistic coefficients atmospheric pressure and a few other varibles that I don't recall off the top of my head, you can precisely calculate bullet estimated trajectory. Of course, when you start shooting, you will very likely discover a variance between the observed value and the expected value, which you could the use to determine the standard deviation and then calculate an appropriate chi-square value to see if your results were within an acceptable confidence level.

But I'd rather go shooting, and then adjust my loads empirically to get the result I want.

I agree with that.
I wasn't looking to make the light load I mentioned shoot to the sights. I didn't expect it to. I was pleasantly surprised when it did.
Then, of course, I had to figure out why.

[Drm50]

Is this based on both projectiles traveling at same velocity?

[DeadWoodDan]

Just finished shooting some loads worded up for M15 & RBH in .38spc. Now I know what can be done, couldn't be a more perfect time for this post Thanks!! DWD

[35remington]

Worth mentioning.

Resting a pistol with the butt on a firm surface or in the hands with the hands resting on a firm surface does not duplicate point of impact as when the pistol is allowed to recoil freely unsupported (as when forearms are rested on bags and pistol is held beyond them in the hands) so conclusions as to resulting points of impact can be invalid as compared to when the pistol is allowed to recoil more freely. A 1911 slide and barrel starts moving backward immediately upon firing and well before the bullet leaves the barrel.

Increasing bullet velocity with the same weight bullet increases recoil arc but reduces barrel time, so these often offset. Generally pistols are sighted for a particular bullet weight and reasonable changes in velocity with the same weight bullet have little deviation in point of impact. If point of impact differences occur with same bullet weight at reasonable differences in velocity they are often more due to the user cinching down (or not) on their hold due to increased recoil or a change in grips used as much as anything else.

[pettypace]

Well, here are the results of an unintentional "experiment" that managed to eliminate most of the variables mentioned. The target below shows five shots from a two-projectile .38 Special load fired off-hand at 5 yards. All ten projectiles were the same 95 grain Lee TL356-95RF bullet. That eliminates the weight variable. The bullets were loaded base-to-base in the cases. So the back bullets made the clean wadcutter holes while the fronts made the smaller holes. But each pair of bullets had identical velocities and identical recoil. Yet, even with the same weight, same velocity, and same recoil, the back bullets showed noticeably higher POI, even at just five yards.

260730

Apparently, the back bullets leave the barrel just an instant later than the fronts, and in that instant, the gun has recoiled enough to significantly raise the POI.

[Silver Jack Hammer]

Win94ae, Thank you for putting this video together and posting it. Your get-to-itiveness really makes castboolits the place for good discussion and learning

[Low Budget Shooter]

Thanks, rintinglen!