Apparent over pressure from mild load

[BongEyes]

G'day all

Im scratching my head on this one. Need some assistance.

Running a 6.5 PRC
28" Custom barrel SAAMI Chamber (.533 web)
on tikka t3 action.

153.5 Berger Hybrid Target
ADI 2217 (H1000) at 50.5grn (4.5grn below book max)
Surplus Indian Army LRP
Norma Brass trimmed to 2.015"
Loaded neck OD = .2935
Shoulder bumped back 0.002" after full length resize using SAC bushing die and mandrel
COAL 3.05"
Approx 25 thou jump to lands
Case capacity measured to 70.5grn h20

Velocity measured to 2915fps via Xero chrono which is super fast at this charge weight according to the book...
Using GRT this load is showing a pretty mild pressure curve...and certainly well below max


But, I have been getting excessive pressure signs; from the minimum charge upto 52.5grn
Heavy bolt lift/clickers
Cratered primers
Blown primer x2 once with a stuck case and torn rim from extractor claw
Ejector swipes
Primer pockets lose/unuseable after 2 firings on some pieces of brass.

Before you crucify me for running over pressure I have tried 2 different bullets and 2 different powders starting low and working higher. All showing signs of OP. 50.5grn has been fairly stable except for the clickers.

Really the only other variable to change is the brass (I've heard Norma can be very hit/miss on its QC and often produces soft brass) and I am heavily leaning towards this as the problem I am facing and wondering if anyone else thinks could be the issue...
However it dosnt explain the vast increase in velocity I am recording compared to what the book is saying...

Any insight on wtf is going wrong with this would be helpful. I have thought of everything I know and will be first to admit I don't know everything.

 

[RegionRat]

I realize it may be difficult to obtain any factory ammo for your rifle, but if you had a factory ammo for a reference load, you would at least know which part of the troubleshooting to investigate.

If that isn't possible due to the rarity of factory 6.5 PRC ammo, then I would inspect the bore and chamber. See if the barrel and chamber are really to the correct specs and also take a look to make sure it isn't copper fouled.

Any decent gunsmith should be able to cast the chamber and throat and also slug the bore and look for fouling.

Velocity (muzzle energy) is a direct clue to the pressure and when it is too high then so is the pressure. Good Luck and in for the reports.

 

[KnowNothing256]

How long is the barrel in the load data? Agreed that you seem to have a pressure issue, but it's possible that your high MV could just be due to extra barrel length, and the pressure signs are another issue.

Other possibilities I can think of:
- Lube on the cases leading to slippage (might not explain the blown primers)
- Carbon ring inhibiting initial bullet movement (spikes initial pressure)
- Tight bore (increases entire pressure curve)
- Incorrect powder weights due to scale reading incorrectly
- Primers are too hot (I'm skeptical of this, but it's possible)

 

[BobinNC]

ADI powder equivalent's are only accurate to within 5% (or 2.5 grains in your case)

Powder equivalents

This chart shows ADI 2217 as being slower than H-1000

Relative Burn Rates​

In other works there is no clear answer as to whether ADI -2217 is exactly the same as H-1000.

 

[Maurygold]

you're 300 FPS too fast for that load. something is up with the barrel/ throat/ chamber

Agree with the above - get a factory round and shoot it to confirm this is the case and it shoots several hundred FPS over the boox rec.

but also - get real primers for the future (this is not causing the issue though)

 

[918v]

What is the fired neck diameter?