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:
131910

Here's a group with the 358477 over a light charge of W231. It seems to be hitting right about the same place.
131911

Now here's the puzzler: groups with the 150-grain round-nose bullet over the same light charge of W231 hit higher.
131912

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.
131913

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.

[1Shirt]

As usual 44Man hits it!
1Shirt!

[35 Whelen]

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.

Physics. Check it out sometime.

Similar weight bullets at similar velocities fired out of the same handgun produce similar recoil/barrel rise. As already stated, the difference in POI at 7 yds. would be indiscernable. You may continue to argue, but that won't change the facts.

[RoyEllis]

Physics. Check it out sometime.

Similar weight bullets at similar velocities fired out of the same handgun produce similar recoil/barrel rise. As already stated, the difference in POI at 7 yds. would be indiscernable. You may continue to argue, but that won't change the facts.

And according to physics, a bumble bee has insufficient wing area to generate the lift necessary to fly. Fact is, different bullet bearing length changes barrel dwell time which can and does affect internal ballistics, as well as the timing of muzzle exit during the recoil cycle...therefor affecting POI.

[robertbank]

^^^ 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

This! Difference is more likely due to sight picture than anything else at that yardage, Shoot the target a dozen times and then compare results. Run the loads over a chrony. Let's not over think this, the difference in POI is not significant. For self defense at 7 yards or at 25 yards the BG won't notice the difference.

Take Care

Bob

[Animal]

Guys, I retract my use of the term "barrel harmonics". I'm sure I used it far too loosely. Thanks

[Fishman]

Physics. Check it out sometime.

Similar weight bullets at similar velocities fired out of the same handgun produce similar recoil/barrel rise. As already stated, the difference in POI at 7 yds. would be indiscernable. You may continue to argue, but that won't change the facts.

It seems to me that the facts are on display in the first post.

The ballistics is not the issue. POI and drop differences between 125 and 158 gr designs out of a pressure barrel at 25 yards is not significant, yet POI from a handgun can differ by 6". You are citing the wrong facts to back up your statement.

[robertbank]

Fishman the OP is talking 7 yards with the same weight of bullet not 25 yards. I am sorry but at that distance I can't see where the POI would be that much different at that distance. Any difference would indicate to me the higher bullet would have less velocity than the lower striking bullet. That said, I still would bet it is a sighting difference. Nothing more nothing less.

Take Care

Bob

[Fishman]

Bob, I agree with your statement about the higher strikes being due to lower velocity. But the center of those two groups are only an inch apart. That wouldn't come close to 6" at 25 yards would it? More like 4". I think different bullet styles resulting in different bearing surfaces (as 44man said) could cause that. Experience aside, looking in the Lyman cast manual reveals different powder charge ranges for similar weight bullets.

[robertbank]

Looking at the two bullets the TC bullet appears to have more bearing surfaces which should translate into more friction and slower velocities, all else being equal. If that is true then it should have hit higher than the RN bullet but it didn't. I really do think it is just a case of sight alignment. The fact we have one result to examine instead of say 25 suggests to me we don't have enough data points to come to any real conclusion. Shooting off a pistol rest I have got careless with one round out of a group of six when dialing my sights on my GP-100 at 10 yards and had a bullet be left of the group by an inch or two. It is easy to to do especially if you are using FO or sights with a white dot alignment. We humans do have our failings.

Take Care

Bob

[Low Budget Shooter]

I sure do appreciate all the information. I went out and shot this again. There is no doubt about it: as unlikely as it seems, the 150-grain RN bullet is hitting about 1" higher than the 150-grain SWC bullet at 7 yards. This has happened consistently on 12 targets over the course of 3 shooting sessions. I know it doesn't make sense; that's why I've posed the question here. I find load development with cast bullets very interesting!

[.22-10-45]

Don't complain about this..it's what allows a handloader with a fixed sighted firearm to find that perfect load that shoots to the sights without resorting to a file!

[Low Budget Shooter]

Right!

[kweidner]

Just for thought. BC would also be a factor in how it overcomes resistance to air. This could be part of the equation too. The flat meplat on the SWC would have more resistance than the RN. Factor this into bearing length and recoil due to pressure of different bearing lengths you have at least part of the picture.

[robertbank]

Run the loads over a Chronograph. Let us know of the results.

Take Care

Bob

[Low Budget Shooter]

Bob, you're right, of course. Somehow a chronograph has never made it to the top of the money priority list.

[44man]

The boolit with more drive band is actually faster so it hits lower. More pressure build up and better burn.

[robertbank]

If there any IDPA/USPSA clubs in your area check them out. Someone will have one for sure and knowing IDPA shooters they will be more than happy to meet you at the range to run your cartridges over the Chrono. From your pictures they really are not that far off each other. In general my experience with cast bullets tells me LSWC bullets are usually more accurate than RN but that isn't what we are talking about here. Given both bullets are the same weight the top shooting bullet is travelling slower which would account for it hitting the target higher. Do you seat the bullets the same depth? The LSWC looks longer which means it would seat deeper into the case which would raise pressures higher than the RN IF both bullets are seated to the same OAL. Higher pressures should mean higher velocity given the bullets weigh the same. The RN would not seat as deep in the case, which should produce a slightly lower pressure which would reduce it's velocity. Lots of variables but I guess those would be the most significant.

Just guessing

Bob

[44man]

BC and air resistance does not come into play, only barrel time does.

[gwpercle]

Here is the truth! Nobody really, really knows. It's just the way things are.
In theory there should be no difference, but in practice.....you not going to know where them boolits is going to land until the targets are in.

Gary

[10mmShooter]

Kweider beat me to it...the BCs are different for those two bullets, this too would have an effect on POI(all else being equal) although small at pistol ranges

[robertbank]

BC and air resistance does not come into play, only barrel time does.

At the distance of only 7 yards this is certainly correct. Deeper seating of the longer bullet will increase pressure which in turn should produce more velocity. Higher velocity and the bullet will hit lower due, as you point out the faster bullet will be in the barrel for a shorter period of time and be less affected by the rising barrel under recoil.

Take Care

Bob

[Low Budget Shooter]

Well, it looks like everyone who is interested has had his say, and I appreciate very much all the information. LBS

[jetinteriorguy]

I do have one question. Why practice with the different bullet? Please don't take this the wrong way, I'm not trying to be a smarta$$, I'm just curious.

[Low Budget Shooter]

That's a good question. The answer is that the hollowpoint mold I have is wonderful, but it takes about 5 times as long to make a batch of good bullets with that two-cavity hollowpoint mold as with a four-cavity solid mold.