What causes case head separations, other than the crappy stepped brass?

[sasquatch981]

I had a bone stock RRA 9mm upper that I ran in local rifle matches for almost 9-years, and never had a single case head separation.  I built a new 9mm AR Shorty upper, and had sevearl case head separations while figuring the system out.  That is when I learned about the internal stepped brass.  Now, when I prep my brass, I tumble, and put everything into a a dillon 9mm bullet box with the open case mouth facing up, and then shine a flashlight over the tops and pull all the stepped stuff out.

 

Now with good brass, and what I was told was the secret sauce is recoil parts, I set about getting ready for the Washington State USPSA Championship.  I went out to test before the match, and zero the dot.  6-round into the session case head separation.  I forgot my tools at home, so had to leave the range.  Got everything cleared at home, and gut the thing zero'd and ready for the match.  Shot 1 stage and had a few FTE's, but went fine.  Second stage 7 shots in, and jam, rack it out, jam, rack it out, jam, look inside and low and behold another Case Head Separation!!!!!!

 

Since building this shorty,  I have had 10+ case head separations, 6 of which could be attributed to the stepped brass.  What else could be causing it?

 

I am shooting 147's loaded to 1.1.  Power factor is right about 135, so nothing super hot.  8.5" wilson 9mm barrel, and FoxTrot Mike Glock bolt with weight removed.  The secret sause I was told for recoil was a JP .308 Spring, (short stroke spacer i.e. delrin spacer measured by use of quarters)  Endine hydraulic buffer, and the waffle spring.  All in a carbine tube.

 

So, what says the collective?

[Ming the Merciless]

What happens when you run the same batch of ammo in the RRA?  If you get case head separations it's the ammo not the carbine.  If no separations, will the bolt and recoil system from the RRA fit into the shorty and vice versa?  If so, what happens when you swap them.  If the case head separations stay in the shorty, start looking at the barrel chamber dimensions.

[stick]

Case head separations are usually caused by two things.

1) stepped brass or 

2) crappy cheap brass

 

That's why I sort my brass.  I only keep the major brands.  Also do you full length size?  you might be getting bulged brass or picking up 9 major brass.  Just a thought.

[sasquatch981]

I don't have the RRA anymore, so I can't speak to that.  I am thinking this just has to be bad brass.  I'll make a note of what headstamps I am using, or just buy factory for bit to see if this solves the problem.

 

[egd5]

How do you get the brass out? A friend just had this happen today and we couldn't get it out.

[sasquatch981]

I can't remember the size of the tap, but basically, I use a tap, that is attached to a long extension, and then from the rear with bolt removed, insert the extension/tap, give about 1/4-1/2 turn just enough to "grab" the case, and pull out.

[egd5]

ok, thanks.

[Darqusoull13]

You can run into a problem with too little mass. Running your bolt "with the weight removed" might not be how the system was designed. I don't like mixing and matching operating system parts for that reason. 

My secret sauce is a JP 9mm bolt with a JP Short Stroke Silent Captured Spring, either in a 5 stainless or 3 stainless / 2 tungsten configuration. All the other SCS setups work as well, but these are what work really well for my particular setups / loads. 

[Aircooled6racer]

Hello: Did you plunk test your ammo in the barrel? It maybe an overpressure problem from a short throat in the barrel. Is the bolt going fully into battery? I have seen a JP buffer setup not allowing the bolt to go all the way forward under spring tension since the buffer tube was a little too long inside. Two thin Delrin shims behind the buffer solved the problem. Thanks, Eric

[1911luvr]

I can't remember the size of the tap, but basically, I use a tap, that is attached to a long extension, and then from the rear with bolt removed, insert the extension/tap, give about 1/4-1/2 turn just enough to "grab" the case, and pull out.

9mm tap. Remove all cutting threads except the one’s that grab the case, just to make sure you don’t accidentally scratch anything.

 

 

 

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[JsK]

The bolt is opening to soon. You need to go back to running  weight in the bolt. Light weight short stroked setups are the most difficult to tune properly. The heavy blitzsetup is way easier to tune and you won't be blowing cases apart.