375 cheytac case head separation

[Pandaboo]

First round through the rifle was 130 grains H50BMG pushing a 377grain MTAC. Velocity was 2688fps. Stable bullet but case head separation and flattened primer. 215 federal magnum primers and Peterson brass.

I planned to work up to ~140 grains but wanted to start conservatively. Was wondering if starting too low caused the high pressure.

JJ rock super xl action with 30” taccomhq barrel. Chamber cut my mirage ULR. Likely more rifle than I deserve. Would have loved to use Retumbo but this build has already taken year and a half to complete. Waiting for Retumbo to be in stock would kill me.

Anyone have experience with h50bmg and 377 grain seeds? I’ve heard ‘it’s too slow’ but I’ve also heard of people running it.

 

[England]

I haven't loaded/shot CheyTac. But I did run into this with a .338 Edge. Problem was me--when using a .338 RUM die to expand the case mouth from .300 to .338, I inadvertently set the shoulder back a bit. This created excessive headspace and resulted in flattened primers and a case head separation. You might check your sizing die.

I now use a mandrel for expanding the case necks of .300 RUM cases to .338 Edge, and also have a case gauge. And if you find Retumbo, please let me know!!

 

[918v]

Did the case head separate completely or can you still measure the shoulder length to compare to a virgin case?

 

[Pandaboo]

Did the case head separate completely or can you still measure the shoulder length to compare to a virgin case?

Complete separation! Extra long shoulder would mean chamber out of dimensions?

 

[Pandaboo]

I haven't loaded/shot CheyTac. But I did run into this with a .338 Edge. Problem was me--when using a .338 RUM die to expand the case mouth from .300 to .338, I inadvertently set the shoulder back a bit. This created excessive headspace and resulted in flattened primers and a case head separation. You might check your sizing die.

I now use a mandrel for expanding the case necks of .300 RUM cases to .338 Edge, and also have a case gauge. And if you find Retumbo, please let me know!!

Make sense! I have heard of that happening. I did check shoulders before and after pushing the expander ball through. Obviously I’m going to go through and double check everything again. My first guess is the low powder charge.

I will let you know if I find some! Thank you for the post!

 

[918v]

Complete separation! Extra long shoulder would mean chamber out of dimensions?

Yes