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06-16-2017, 03:25 PM
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Ammo Puzzler with Model 27
I have an ammo puzzler with a Model 27 (no dash) that I recently got. I have the gun sighted for .38 Special 154gr FMJ (Magtech and S&B, so far) to give me an impact point about 1 1/2 inches above point of aim at 15 yards. When I shoot .357 Magnum through the gun (Geco 158gr FMJ, so far), I get a point of impact at 15 yards about 5 inches above point of aim and about 4 inches left. The rounds are grouping, just at a very different impact point. I’ve shot .357 Mag through my Model 19, Model 66, Model 28, and Model 686 with about the results you would expect: a slightly lower mean point of impact without any drift left or right, so I'm puzzled by the extreme difference I'm seeing with the Model 27. The Model 27 looks to be in excellent shape, with good lock-up and a shiny bore. I had a gunsmith check the timing, and he said it was fine. I have not had the cylinder alignment checked, but I’m not seeing signs of a major misalignment issue. I did have to crank the rear sight left to get the .38 Special rounds centered on the target, so I suspect that centering the rear sight would center groups with .357 mag. Any thoughts as to what could be causing such a large and consistent shifting of the impact point between .38 Special and .357 Magnum? Have others seen the same thing?
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06-16-2017, 08:38 PM
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POI has everything to do with bullet velocity. And it can also be affected by the weight of the bullet if the velocity is close to the same. Dwell time in the barrel changes the height of the rise when the gun recoils. The same cartridge in a 2", a 4", or a 6" barrel (or longer) can affect the POI for these reasons.
A particular cartridge will only shoot to your POA (if the sights are regulated properly) at one shooting distance. Bullet trajectories are more like a rainbow as opposed to a straight line because of the force of gravity on the bullet and the time it takes that bullet to hit the target after it leaves your gun barrel.
You are comparing two very different cartridges with very different velocities even though the bullet weights are very close to the same. The POA will be different for those two cartridges! My answer here is only very general in nature. Many things can affect these variables, but I think this information will help to answer your question. I certainly stand to be corrected here, and this answer can certainly be improved upon!
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06-16-2017, 08:53 PM
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Don't forget, also, that the two different cartridges will create different patterns of barrel and frame vibration. It may well be as simple as the muzzle being in a different place when the bullet leaves. Simple ? Easy to say, pretty tough to correct by any means except extensive load development. As noted above, different load usually = different POI.
Larry
Last edited by lebomm; 06-16-2017 at 08:54 PM.
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06-16-2017, 11:34 PM
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Not exactly a correction - more of an observation, but....
It is true that the path of the bullet from barrel to target is an arc due to gravity pulling the bullet down towards the ground. However, at 15 yards the amount of drop due to gravity is going to be very tiny. At that short of range the bullet's trajectory is going to be almost perfectly flat.
There is also some effect from gyroscopic oscillation. The bullet is spinning at high RPMs and minute imperfections create both a slightly out of balance wobble and aerodynamic forces that disturb the bullet's flight path. Think of how a spinning top does not stay in one place but moves around in little circles (oscillates). The bullet does much the same thing - only in 3 axis of motion (left/right-up/down-forward) vs the top's 2 axis of motion (left/right-forward/backward). Kind of like how a spiraling football doesn't fly in a perfectly smooth arc - but instead sort of corkscrews a little bit. But that is also only a minor variation.
The big variable is the movement of the gun under recoil. Barrel rise, and "torquing" sideways due to grip, etc. The fact that the shots are still grouping despite the difference in POI it the key proof of it. It is totally repeatable.
Last edited by BC38; 06-17-2017 at 12:06 PM.
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06-17-2017, 12:22 AM
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If you want to see variations where these effects are dramatic.....
try shooting a K-frame 4" Model 16-4 in .32caliber....
1. .32 S&WL 100 grain wadcutter @ 600fps
2. .327 Fed. 100 grain Hornady XTP @ 1300fps
It is remarkable to see the groups...
If the wadcutters are center of bullseye at 50 ft.
the .327 Federal loads group 5" left and 6" high.
I expected that the .327 loads would group LOW, but go figure....
This is repeatable with a Ruger single 7...
the groups are fantastic with either load...
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06-17-2017, 08:16 AM
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"Revolver A shoots ammo B like this; Revolver C shoots ammo D like that. Why?" My answer, "Why not? Different guns, different loads, different results."
Rules of thumb, such as heavier-faster loads shooting higher than lighter-slower loads, are useful generalities, but nothing more. They are not Gospel, and exceptions to the rules abound.
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06-17-2017, 08:45 AM
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I doubt it is a mechanical issue. I presume you are shooting it from a solid rest; have others shooting the 27 with the same Geco load gotten the same results?
I would bet a different 158 gr .357 load (slightly different bullet diameter, bearing surface, shape and velocity) will print closer to the sights.
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06-17-2017, 04:30 PM
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Groo here
The biggest thing that effects revolver impact is your grip.
The more you allow the gun to move the more change.
Does the m-27 have different grips?
If so they don't fit your hand like the others and the shifts
will show weakness/difference in your grip, more so with heaver loads.
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06-17-2017, 09:53 PM
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Thanks for all the responses. I'm shooting offhand, but getting groups in distinctly different places over 200 rounds of shooting in similar conditions. The point about .327 mag versus .32 is interesting, and sounds very similar to my results. All the guns I mentioned have S&W target grips. The M-27 has a 5 inch barrel, the M-686 has a 6 inch barrel, and all the others have 4 inch barrels--so to the comments above, none are exactly equivalent to the M-27. I guess I chalk this up to the characteristics of this particular gun, but perhaps try some other .357 loads to see how the point of impact changes.
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06-27-2017, 03:39 PM
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Hello slowguy, just came across this discussion. I have a Model 27 with 5" barrel. Shooting from a rest as carefully as I can, the 27, or any of my other guns, will group up/down/left/right, etc with different loads. If I get two loads that group the same at 15 yards and beyond, that is unusual for me. Also, within my humble experience there is only a casual relationship where two similar guns, using the same ammunition, group for me. When I sight in any of my guns with a particular ammo, I know that any other ammo, even with the same bullet weight, will likely group differently......ymmv
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