Pressure at Starting

[deertroy1]

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.

Possibly but if that's the case my barrel is really slow. My 22" barreled Savage gets 3700 fps with no signs of pressure. Hodgdon gets 3710 fps with a 24" barrel. Quickload shows my hot load at 45000 psi?

 

[Shawnba67]

@deertoy1
I had a 6xc doing the exact same thing to me once. It turned out to be soft brass! I was using new Norma brass, had ejector swiping at start loads and all. I nearly scratched my head raw!

 

[LCazador]

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.

This indicates a short chamber and headspace issue. I've experienced this with several Coopers. Savage and Cooper are not the same. Contact Cooper and let them resolve the issue.

 

[Webster]

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?

My Berger manual has the following for 4064 powder.
69 gr bullet start 37.5 3119 fps max 41.6 gr. 3428 fps
80 gr bullet 36.5 gr 2997 fps max 40.4 gr. 3279 fps

The Berger manual gives the highest fps with 4350, 4831 and W760.

Sierra manual 5th. Edition
70 gr. bullet start 38.5 3000 fps max 41.9 3300 fps

I always shot 4350 and 4831 in my 6mm Rem. I shot a lot of 75 & 85 gr Sierra bullets. Rebarreled to a 6BR now. I would switch to 4350 and just accept whatever I get. Go for the best groups and don't worry about speed as long as it's reasonable. Cannot imagine what tenon strength has to do with ejector swipe marks. Sounds like a varmint rifle. Any group under 1" is good enough to kill GH out to 300 yds. The 6mm Rem is hard on barrels. Some manuals are conservative. Hornady has a starting load of 40.0 and Sierra 38.5. Good hunting!

 

[Webster]

This indicates a short chamber and headspace issue. I've experienced this with several Coopers. Savage and Cooper are not the same. Contact Cooper and let them resolve the issue.

Don't understand the short chamber related to pressure. The OP stated it was checked with a HS gauge. If you can easily chamber a case how does it fit in a short chamber?

 

[Bill K]

This indicates a short chamber and headspace issue. I've experienced this with several Coopers. Savage and Cooper are not the same. Contact Cooper and let them resolve the issue.

I can't believe Cooper would send rifles out with headspace issues. Always thought they did much better work than that. Heavens they even include a target with the rifle sale, that shows how tight a group it shot, even if it is only at 46 yrds.

 

[243winxb]

OD of loaded rounds neck? OD of fired neck of brass that shows ejectory marks?

Thinking the brass is not releasing the bullets. Just a guess.

 

[wagonman]

This indicates a short chamber and headspace issue. I've experienced this with several Coopers. Savage and Cooper are not the same. Contact Cooper and let them resolve the issue.

op stated he checked chamber with headspace gage

 

[Hoot]

I'm a poor reader but did you say what brand of brass you're using? IE Maybe Cooper used a chamber reamer based upon a thinner walled brand of case. Some of rising star cases out there are fairly thick walled. just a thought...

Hoot

 

[deertroy1]

The brass is not soft as the same brass in the Savage can be loaded with 3 - 4 grains more powder and 300 - 400 fps faster.
Neck thickness is not a problem. The necks were cleaned up with a PMA neck turning tool and a bullet drops in a fired case with room to spare.

 

[DiffEQ]

I would caution on comparing your head space measurement to the savage: two people measuring chambers on different guns does not give a frame of reference. Both need to measure both chambers for a valid comparison.
Your gunsmith's comment on "small tenon causing higher pressure" sounds like a load of hooey.
I'd suggest doing a chamber cast and slugging the bore.

 

[Jager]

Possibly but if that's the case my barrel is really slow. My 22" barreled Savage gets 3700 fps with no signs of pressure. Hodgdon gets 3710 fps with a 24" barrel. Quickload shows my hot load at 45000 psi?

While it's true that QuickLoad will give you default numbers, and it's tempting to think those numbers are accurate - sometimes they are, and sometimes they're not. Until you've calibrated QL to a particular rifle/powder/bullet, it's not the refined tool that it is once you've done that. The 45K psi it's reporting might be right on the money for your Savage, but entirely out to lunch for your Cooper.

What gage(s) did you use when you verified headspace?

Assuming headspace is within spec and cartridge case is not over-long, my guess would be tight chamber and/or tight bore. Either or both can be "within spec" but still on the tight side. Doing a chamber cast is a pain in the ass, but slugging the bore is pretty easy.

It'd be interesting to see what Cooper has to say, as well.

Good luck with it.

 

[LCazador]

This indicates a short chamber and headspace issue. I've experienced this with several Coopers. Savage and Cooper are not the same. Contact Cooper and let them resolve the issue.

A chamber is measured from the case head face to the mouth of the case. Headspace is measured from the case head face to the shoulder datum. One thing I'll add is the fact that the OP used the same case in his Savage rifle and he had 3 grains extra capacity. That indicates a short chamber in the Cooper in my opinion.

 

[AlNyhus]

From my experience with IMR4064 in the 6MM Remington...that's a long way from a starting load in that case with that bullet weight. Around 38.0 is what my notes show I've used as a starting point.

Try another appropriate powder and see how it acts. -Al

 

[dellet]

It might be worth checking the bore size the entire length of the barel. Slug the bore at minimum is easy.

You already know that the throat is shorter on the high pressure barrel. If the first few inches of the bore is tight, and then loosens up, you have a swagged down bullet traveling into a loose bore.

That would explain high pressure signs on the case, low overall velocity, and accuracy drop off after a few rounds. If i understand all the symptoms correctly.

Pushing a bullet or soft lead slug through the entire length will allow you to feel a loose spot.

 

[deertroy1]

A chamber is measured from the case head face to the mouth of the case. Headspace is measured from the case head face to the shoulder datum. One thing I'll add is the fact that the OP used the same case in his Savage rifle and he had 3 grains extra capacity. That indicates a short chamber in the Cooper in my opinion.

No I did not have any extra capacity. The capacities are almost equal. I said I can load three more grains of powder in the Savage without any pressure signs!

 

[deertroy1]

While it's true that QuickLoad will give you default numbers, and it's tempting to think those numbers are accurate - sometimes they are, and sometimes they're not. Until you've calibrated QL to a particular rifle/powder/bullet, it's not the refined tool that it is once you've done that. The 45K psi it's reporting might be right on the money for your Savage, but entirely out to lunch for your Cooper.

What gage(s) did you use when you verified headspace?

Assuming headspace is within spec and cartridge case is not over-long, my guess would be tight chamber and/or tight bore. Either or both can be "within spec" but still on the tight side. Doing a chamber cast is a pain in the ass, but slugging the bore is pretty easy.

It'd be interesting to see what Cooper has to say, as well.

Good luck with it.

I calibrated Quickload for both rifles. This included weighing ten fired cases for each, as well as calculating the weighting factor and adjusting the BA value of the powder to match actual velocities recorded on the Oehler 35.

 

[deertroy1]

From my experience with IMR4064 in the 6MM Remington...that's a long way from a starting load in that case with that bullet weight. Around 38.0 is what my notes show I've used as a starting point.

Try another appropriate powder and see how it acts. -Al

I started at 39gr, Hodgdon started at 40gr

 

[deertroy1]

I would caution on comparing your head space measurement to the savage: two people measuring chambers on different guns does not give a frame of reference. Both need to measure both chambers for a valid comparison.
Your gunsmith's comment on "small tenon causing higher pressure" sounds like a load of hooey.
I'd suggest doing a chamber cast and slugging the bore.

I measured the headspace on both rifles. My dies are set to bump shoulders no more than 0.002"

 

[AlNyhus]

I started at 39gr, Hodgdon started at 40gr

How did it act with another powder?