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?

 

[BongEyes]

Really appreciate all the replies so far. Hopefully I can get to the bottom of this with every ones help...

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.

My initial 20 rounds were from a box of factory Norma Whitetail 140gr soft point, used for breaking in the barrel and sighting in the scope.
Box velocity is listed at 2887 FPS (I assume 24" barrel)
I got a solid 3000fps average from the entire box of 20.
I cant say that these rounds "felt" right from the sound of firing to the feel of the recoil and even the slightly heavy bolt lift. It definitely felt odd to me. Looking back at the fired cases there is evidence of ejector swipe on the head stamps. I had not noticed this prior to just now.
Measurements of these cases are
AVG fired neck OD 0.297
AVG fired OD at web 0.533
AVG fired OAL 2.025
Unfortunately the gunsmith who built the rifle for me has been far less than helpful (practically useless) at trying to help me fix this issue. But I agree, I think casting the chamber and throat would be a good idea.

As for fouling, I have a pretty good cleaning regime. I usually clean every 50-100 rounds with good quality chemicals and equipment. I am mindful to work on the throat to avoid carbon rings, although I have no borescope to confirm this - I am fairly certain there wouldn't be a carbon ring present (I could very well be wrong of course)

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)

Load data barrel is 24"

-I use Redding die wax, and always make sure to wipe the excess after resizing. I clean the chamber of any excess oils before firing and store my guns muzzle down so nothing creeps back into the chamber. I am pretty good when it comes to this.
-Carbon ring could be a real possibility... I am good with rifle cleaning and maintenance but don't own a borescope so cannot confirm that it exists or not... I think I will have to take to gunsmith to take a look.
-Tight bore is also something I would have to take to gunsmith to inspect.
-I did think about the scales being incorrect, but the level of powder in the case leads me to believe it isn't throwing too much powder. I have used these scales for many years and trust them. BUT I will check them just in case.
-Hot primers did also cross my mind, and they are an unusual purchase for me as Id usually use CCI br2 or br4 primers but these days with components I have to get what I can take. Funnily enough under initial testing I have had really low and consistent S.D's which leads me to believe they are ok?

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.

Those are some useful links Bobin, thanks for sharing.
I know that 2217 and H1000 aren't exactly the same but fairly similar. The data I am working with is listed straight from ADI's website at 49.5 min to 55.0 max with a 153 grain bullet.
H1000 was just for reference because Im not sure too many people would understand what I am talking about if I said AR2217.

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)

I agree, something is definitely wrong.
As above, factory rounds through my rifle were ~115 fps faster than box. Assuming the box data is based on a 24" barrel that seems like a fairly normal increase for a 28" barrel?
Primers are subject to availability... Where I live, there is little demand for shooting components so we tend to get everything last. Im trying to make the best out of what I can obtain.

What is the fired neck diameter?

Factory loaded brass 0.2965 to 0.297
My handloads 2x fired and annealed 0.296 to 0.2965


Seems like the next logical step is to have bore inspected for carbon ring, and the chamber/throat inspected and cast...