Pressure at Starting

[deertroy1]

I have a Cooper M54 in 6mm Rem. that shows signs of pressure a grain or so above published starting loads. I tried a couple different powders and same thing. Velocities however are close to starting load velocities (Oehler 35). The barrel had a carbon ring just ahead of the chamber when I got it which I thought was causing the pressure issues but after cleaning it out (still waiting for borescope to confirm) the issue remains.

Example:
Hodgdon starting load 24" 40gr IMR 4064 Sierra Bullet = 3355 fps

My load 24" 40.5gr IMR 4064 Sierra 70gr Bullet = 3262 fps shows ejector mark

Same load in my 22" McGowen barrelled Savage was able to reach Hodgdon max of 44.5gr and over 3600 fps no signs of pressure. Same components, same brass. Savage has a slightly longer throat but both loads are nowhere near the lands.

I've been loading for 30 years and this one baffles me!

Ideas?

 

[chromatica]

What caliber?

 

[deertroy1]

What caliber?

Edited post above.

 

[Bill K]

Tight chamber or maybe cartridge neck is too long and needs trimming.

 

[dkhunt14]

It still could have carbon in the throat. That stuff gets really hard and sometimes takes days to get out. Matt

 

[EOD Rob]

Tight chamber or maybe cartridge neck is too long and needs trimming.

One would think he would has issues with the bolt if that was the case, right?

 

[Bill K]

One would think he would has issues with the bolt if that was the case, right?

Maybe. but then maybe not, other than some slight more pressure to close it on a reloaded case.

 

[rijndael]

How far off the lands?

 

[deertroy1]

I'm guessing it's still an issue with carbon ring. So far, the first three or four rounds group nicely and then things open up.
Bullet is about .010" off lands. Trim to length is good.

 

[EOD Rob]

Maybe. but then maybe not, other than some slight more pressure to close it on a reloaded case.

Makes sense Bill, thank you. That was a serious question! Anyone think it could be a headspace issue? I'm not sure what it is, just throwing ideas out there and learning!

 

[243winxb]

shows signs of pressure

What ones?

 

[Richard Coody]

Did you short stroke the throat and first.10" of barrel with losso or jb to get that carbon ring

 

[deertroy1]

I'll answer a few more questions.

It's not a headspace issue. I measured the headspace with a headspace gauge. The Cooper is within 0.001" of the Savage which was checked by a gunsmith.

The pressure sign is ejector mark on the case head. If loaded higher bolt lift also becomes sticky. The same brass in the Savage can be loaded several grains hotter and 300-400 fps faster even though 2" shorter barrel.

Yes, I short stroked the first 10" or so of the barrel with JB.

 

[D-4297]

From what I've read here so far , I think it may be wise to check the bore diameter / Lands Diameter of the barrel , and to mic the bullets . Just to rule out those two items . Over-length brass can cause a issue , as can carbon ring , but there's something else going on here .

 

[EOD Rob]

I'll answer a few more questions.

It's not a headspace issue. I measured the headspace with a headspace gauge. The Cooper is within 0.001" of the Savage which was checked by a gunsmith.

The pressure sign is ejector mark on the case head. If loaded higher bolt lift also becomes sticky. The same brass in the Savage can be loaded several grains hotter and 300-400 fps faster even though 2" shorter barrel.

Yes, I short stroked the first 10" or so of the barrel with JB.

No issues in the savage, that is what has me! I'm leaning to issues with the rifle not the load.
Oh, I know what it is, stop buying expensive rifles and expecting them to shoot well troy! LOL Stick with the gun you bought for $300, than built and found out that it shoots like a dream like I did! LOL (SAVAGE)
I'm just kidding Sir, I just do not know where to start with this one!

 

[wagonman]

I'll answer a few more questions.

It's not a headspace issue. I measured the headspace with a headspace gauge. The Cooper is within 0.001" of the Savage which was checked by a gunsmith.

The pressure sign is ejector mark on the case head. If loaded higher bolt lift also becomes sticky. The same brass in the Savage can be loaded several grains hotter and 300-400 fps faster even though 2" shorter barrel.

Yes, I short stroked the first 10" or so of the barrel with JB.

its the carbon in front of case neck going into freebore or lead that causes pressure spikes remove your bore guide to be able clean then short stroke patched brush with JB first two inches, as said above it harder to remove than you think if its built up. Everyone by now should own a low cost borescope to verify your cleaning routine.

 

[deertroy1]

its the carbon in front of case neck going into freebore or lead that causes pressure spikes remove your bore guide to be able clean then short stroke patched brush with JB first two inches, as said above it harder to remove than you think if its built up. Everyone by now should own a low cost borescope to verify your cleaning

No issues in the savage, that is what has me! I'm leaning to issues with the rifle not the load.
Oh, I know what it is, stop buying expensive rifles and expecting them to shoot well troy! LOL Stick with the gun you bought for $300, than built and found out that it shoots like a dream like I did! LOL (SAVAGE)
I'm just kidding Sir, I just do not know where to start with this one!

The rifle shoots great (0.25" - 0.5") but always the first group I fire after cleaning. The next couple groups are usually 1" - 1.5"

 

[deertroy1]

its the carbon in front of case neck going into freebore or lead that causes pressure spikes remove your bore guide to be able clean then short stroke patched brush with JB first two inches, as said above it harder to remove than you think if its built up. Everyone by now should own a low cost borescope to verify your cleaning routine.

I've ordered a borescope. Still waiting.

 

[deertroy1]

Gunsmith said Coopers don't handle pressure as well because of small tenon. That could be but 300 - 400 fps below max seems like a lot!

 

[K22]

My Sierra Manual is quite old but it shows the starting powder charge weight for a 6MM Rem, 70 grain Sierra at 37.7 of 4064

Lyman show 37.5 as starting charge weight for a 70 Sierra with IMR 4064.

Is it possible that your load is too hot for that specific rifle?

Perhaps you should start at the Sierra recommended starting powder charge weight and work up slowly.