Why are two equal-weight bullets getting different POI !?!?

[Low Budget Shooter]

Dear Pistoleers,

I'm stumped by something. Would you give an opinion if you think you might understand?

I appreciate the help you all provided in deciding which carry load to use in my 3" Colt Cobra. I settled on the 142-grain 358477 SWCHP over 4.8 grains Unique. Now I'm trying to find a practice load that will have the same POI. Three bullets are candidates, all right about 150 grains: 358495 WC, 358477 SWC, and NOE 360-150 round nose. In test firing at 7 yards, I am having a result I do not understand. Would you help me figure it out?

Here's a group with the carry load. It's hitting right at point of aim, which was the center of the 3-inch circle:


Here's a group with the 358477 over a light charge of W231. It seems to be hitting right about the same place.


Now here's the puzzler: groups with the 150-grain round-nose bullet over the same light charge of W231 hit higher.


The 358477 and the NOE round nose are both right about 150 grains; the 358477 extends down into the case a teeny bit more, just barely; the powder charge is the same; the gun is the same; the shooter is the same; it was the same day, just a few moments apart, and the effect was similar in more than just these two groups.


What, then, could explain the higher POI for the round-nose bullet compared with the SWC bullet?

Thanks!

LBS

[Boolit_Head]

I think you are assuming all things being similar that these loads with different bullets should have the same POI. But it is exactly the differences in these loads and how they interact with your particular firearm that are causing the differences in POI. For one the differences between the round-nose bullet compared with the SWC bullet could cause a POI difference. I think the best you can do is select the closest to your carry load and run with it.

[lefty o]

barrel harmonics. not uncommon with rifles, most people just dont notice it in a pistol, especially up close like that.

[Omega]

^^ +1, and from what I see you can use either one as training ammo or carry ammo on the way home for that matter. Have you tried recovering any of your training rounds? The thing is to be consistent with all three, that is use the same POA for all three and use the grouping to gauge how well you are shooting. All things considered if the 358477 boolits keep landing like that I would carry those.

[44man]

One boolit has a longer drive length then the other and that changes pressure between the two.
The change in velocity and barrel times are different.

[Low Budget Shooter]

Very interesting.

[boltaction308]

I agree with 44man. The internal ballistics are a little different between the two causing slightly different muzzle velocity which gives different poi

[runfiverun]

Bearing length on the boolits.

[35 Whelen]

A slight difference in MV cannot possibly show a discernable difference in POI at the short ranges you're shooting...at least from a exterior ballistics standpoint.
I too have experienced this and cannot explain it. My first incident was with one of my
44 Specials and almost identical bullets; an RCBS 44-250KT and an NOE 429421. At similar velocities their POI's were substantially different at 50 yds. Same in my 45 Colts when switching between RNFP's and SWC's.
I just try to learn the little differences and aim accordingly.

35W

[Low Budget Shooter]

Thanks for all the info!

[vmathias]

Ditto on the pressure statement of 44 mag. A chrono would possibly get you closer to what is actually happening. I would guess slightly higher velocity with the higher pressure load COULD change the POI. From an instructor point for personal defense I'll take the last group.

[MtGun44]

44man has it.

[mnewcomb59]

I have noticed jacketed bullets, at the same weight as a cast bullet, tend to shoot off to the left or right. I assumed the harder engraving of the jacketed bullet resists the rifling, "untwisting" it towards parallel and throwing the bullets off to the side. I'm not explaining myself very well lol but I think the jacketed bullets give more rotational torque against the rifling or something.

[destrux]

Could be a difference in the way each bullet reacts when it engraves into the rifling.

Could be any of the other things people already said too. I don't really know for sure.

[huntrick64]

Just a difference in pressure changes the barrel lift (recoil) enough to hit higher or lower. If the barrel didn't move at all when fired, then you wouldn't notice the point of impact difference due to a slightly different bullet speed (at this distance), but your barrel is moving up as the pressure increases. In essence, the barrel is at a different location in relation to the target when the bullet exits the barrel. I think I am saying what 44man was saying, just in a little different way. Sort of like wondering how a slower rifle round hits higher than a faster one?? By the time the slower bullet exits the rifle barrel, the barrel is higher due to recoil, which would be just like aiming higher on the target.

[Groo]

Groo here
+1 44 man
Barrel time and recoil.
The difference is not great but it is there .
If you shot distance, 25 to 50 yds the change would be much greater.

[Animal]

This is interesting. I'd like to see this phenomenon reproduced multiple times before I'd say there is a difference. Are you getting this to occur regularly? If so, I think I'm in the "barrel harmonics" camp as well. But for a load that is designed for 20yards or less defense, I'm not sure it is a problem.

[shoot-n-lead]

They shoot to different points of impact...because they are different bullets...come on.

[35 Whelen]

This is interesting. I'd like to see this phenomenon reproduced multiple times before I'd say there is a difference. Are you getting this to occur regularly? If so, I think I'm in the "barrel harmonics" camp as well. But for a load that is designed for 20yards or less defense, I'm not sure it is a problem.

^^^ This^^^.

I assure you it's NOT due to pressure, velocity or any such other exterior ballistic figure, not at 7 yards. If you doubt me, run a ballistics program of the bullets in question and you'll find the difference in points of impact at 7 yards in the HUNDREDTHS of an inch.

35W

[44man]

^^^ This^^^.

I assure you it's NOT due to pressure, velocity or any such other exterior ballistic figure, not at 7 yards. If you doubt me, run a ballistics program of the bullets in question and you'll find the difference in points of impact at 7 yards in the HUNDREDTHS of an inch.

35W

Programs are not the real world when a revolver barrel changes rise from recoil. They do not factor in gun weight or barrel length. Harmonics have no affect either with short barrels. It is only barrel time.