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Head scratcher - my production pistol 130PF load make only 110PF in my JP GMR-15?


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On 2/10/2021 at 2:04 PM, Galun said:

I figured it out.  It’s the bullets, 147 gr 0.356 copper plated Xtreme bullets.  Sport pistol, power pistol, long shot, doesn’t matter what powder.  Huge variance in velocity, extreme spread was 150-200fps.  With long shot powder I even got some at 500fps.

 

The plated bullets made me wonder what kind of twist rate you have in the PCC? Also have you trapped the bullets to see the condition of the plating?

 

I suspect it's not the diameter as much as the plating is possibly coming apart. It is my understanding a high/fast twist rate can tear up poorly plated bullets.

 

It's good that you found comb which works.

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14 hours ago, HesedTech said:

 

The plated bullets made me wonder what kind of twist rate you have in the PCC? Also have you trapped the bullets to see the condition of the plating?

 

I suspect it's not the diameter as much as the plating is possibly coming apart. It is my understanding a high/fast twist rate can tear up poorly plated bullets.

 

It's good that you found comb which works.

 

I don’t have the ability to trap bullets.  But yes, now I do suspect that the plating is falling apart with some powder.  When I made that post I haven’t done accuracy testing yet, only chrono.

 

This was accuracy testing at 50 yards.  SP = sport pistol, PP = power pistol, LS = long shot.  Heavy left to right crosswind.  I just had a non magnified holosun 510c so I am sure there is some user induced error here.  But it does give me a good idea on comparative accuracy of the projectile and powder combination.  The optic was zeroed for 147gn velocity so all 115gn groups were ~3 inch high.

 

The 147gn with sport pistol grouped surprisingly well - 8 shots with 3MOA, and 2 shots which I think is user error.  It does make me think the higher diameter of the 147gn isn’t the issue.  Power pistol, which was slower burning, drifted to the right due to wind (POA was at the top right target).  Long shot, which was the slowest burning, drifted to the right even worse (POA was at the top left target).  I suspect the plating with these two powders had fallen apart by the time it left the barrel, leading to an unstable projectile.

 

The 115gn with the slower powder were fine.  JP suggested 3MOA is achievable with their barrel, and I got 4 - 5 with a non magnified dot sight for all loads.  With the 115gn I got the best grouping with long shot which was the slowest burning powder.  

 

The 147gn sport pistol had the best grouping, but it still drifted to the right a bit due to wind (POA was at center target).  The 115gn generally didn’t drift at all, it stayed in the middle in line with my POA.  Maybe the slower muzzle velocity made the 147gn more susceptible to wind?  It’s a difference of around 250fps, only out to 50 yards... but whatever.   Maybe I wasn’t accounting for wind until later.  I just plugged the 147gn into ballistic calculator, at 880 muzzle velocity 10mph crosswind will cause 3.1 inch drift to the right at 50 yards.  It’s the first time I’ve done pcc accuracy test, at my normal AR-15 load the same crosswind will cause only 0.1 inch drift.  Next time I will make sure to feel no wind before I make the shot.

 

D39011CA-678B-4B36-9F3D-ED6425227E04.jpeg

Edited by Galun
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