Questions about 6 BR or BRX in an AR15

E

E_MAC

New member
I was reading the 243 LBC vs 6x6.8 thread and had some questions about some of the posts.

GL SHOOTER said "The 6X68 is faster because of a higher operating pressure"

1. Why can the 6x6.8 operate at higher pressure than the 243 LBC?

Ritch said "The 6mm BR is an impressive cartridge. But it only shines when used in a bolt gun. The AR platform is not capable of running at those pressures. The Super Bolt only address one of the problems. It will take the head thrust but when it comes to Hoop strength the case is simply to big in diameter to push at the pressures of a bolt gun. I wouldn't push it past 35 to 40,000 PSI but that's my opinion."

I know the .223 Rem and 6.8 SPC are loaded well above 40,000 PSI and operates just fine in the AR platform. I'm pretty sure the answer to #3 will answer #2, but I'd like to get my head wrapped around this before I start a build. I'm considering all three of these cartridges and I'm trying to learn as much as I can first. It's amazing how many times you can lean one direction then read more on the subject and change directions again. Mind boggling sometimes!

2. Is this because of the 6 BR AR bolt design?

3. What is hoop strength?
 
Originally Posted By: E_MACI was reading the 243 LBC vs 6x6.8 thread and had some questions about some of the posts.

GL SHOOTER said "The 6X68 is faster because of a higher operating pressure"

1. Why can the 6x6.8 operate at higher pressure than the 243 LBC?

Ritch said "The 6mm BR is an impressive cartridge. But it only shines when used in a bolt gun. The AR platform is not capable of running at those pressures. The Super Bolt only address one of the problems. It will take the head thrust but when it comes to Hoop strength the case is simply to big in diameter to push at the pressures of a bolt gun. I wouldn't push it past 35 to 40,000 PSI but that's my opinion."

I know the .223 Rem and 6.8 SPC are loaded well above 40,000 PSI and operates just fine in the AR platform. I'm pretty sure the answer to #3 will answer #2, but I'd like to get my head wrapped around this before I start a build. I'm considering all three of these cartridges and I'm trying to learn as much as I can first. It's amazing how many times you can lean one direction then read more on the subject and change directions again. Mind boggling sometimes!

2. Is this because of the 6 BR AR bolt design?

3. What is hoop strength?



The reason the 6.8 case can run at higher pressure than the LBC/Grendel is simple. It is because of the size of the case head. Or simply the amount of pressure that is put on the locking lugs because of rearward thrust when the round is fired. The bigger the case head the less material in the bolt. Less metal means less pressure the bolt can withstand

There are bolts out there that can withstand more head thrust. That being said you need to look at what are typical operating pressures of the cases you mentioned. Hodgdon will only give you data with the Grendel case to 50,000 PSI and IIRC 56,000 for the 6.8 case.

Hoop strength is the term used to determine the amount of stress the barrel can withstand laterally. Obviously the larger the case the less hoop strength the barrel or chamber has. Quite simply an AR chamber is limited in size not only from the magazine but the barrel nut as well. You can't simply just make the chamber thicker.
 
Got it, thank you. How does the wssm line work, being such a fat case and the pressure they run, in regards to hoop strength?
 
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