AR15 pistol blowing gas blocks
Hi all,
Im having an issue with my AR pistol project. Basically, I keep blowing small holes in my gas blocks, usually inline with the vertical riser porting (not horizontal inline with gas tube.) The first gas block was a mystery block that came on the upper (CMMG UR/BCG with a citadel (?) 7.5 inch barrel, stainless steel gas tube). It lasted maybe 25 rounds, no rapid fire, no hot rounds. 55gr. Federal ammunition. Then I noticed excessive heat venting back at me and the weapon began short stroking. I took it in to the local AR shop and they found a pinhead sized hole blown up and at a slightly rearward angle through the block. They replaced my gas block with a Troy low-pro block (that also has a set screw on the side) and a new gas tube, and also checked my bolt/bcg and barrel for issues. Took it to the range yesterday, and less than 20 rounds downrange and same issue. This time using .55grain PMC brass ammo.
This is my first AR, and ive searched the whole of Google and cant find any reference to an issue like this. The gun shoots amazingly well, right up until the gas block explodes. The AR shop said my first gas block was a crappy imitation and a good brand would solve the issue. But now my Troy exploded too.
Any input is very welcome!
And just in case its a concern, I have a full length RRA pistol tube, Pistol spring, and heavy buffer. Self made Tactical machining lower, milled to 100% on a CNC machine. RRA parts kit. Troy Claymore muzzle brake.
Thanks for your input!
-L
Not a clue, other that maybe the barrel gas port is too large, and the gas block seeing too much pressure, which is really hard to do since the gas block passage and even gas tube ID are way larger than the gas port to begin with.
Old priest and young priest...
Sorry I'm not any help, I've never heard of this either. Good luck.
Fatkid
You blew/burned a hole through a Troy low-pro block? Steel gas block? What is the diameter of the barrels gas hole?
Keep in mind that the AR15 was designed for a 20" barrel, which places the gas port approximately 12" down the barrel from the chamber. Chamber pressure of a 5.56 NATO round is in the ballpark of 50,000 psi (Don't know exactly off the top of my head - I'm typing this at work and don't have the numbers in front of me.), and sees roughly 16,000 psi at the gas port of a 20" rifle.
When Colt's was trying to get the M4 carbine to run reliably (a 14.5" barrel), one of the issues they had was high gas port pressure (approximately 23,000 psi if memory serves) which caused high bolt velocity and excess rate of fire (hence the M4's heavy buffer) and feeding issues (hence the M4 feed ramps we all know and love). Still today, M4 carbines have a 3 to 6 times more frequent failure rate than do 20" rifles (depending on who does the testing and how they tabulate failures), including bolts breaking at the cam pins.
A 7.5" barrel has to have its gas port a whole lot closer to the chamber than either a 20" rifle or a 14.5" M4 carbine..
Your 7.5" barrel has its gas port a whole lot closer to the 50,000 psi end of the pipe than it is to the 16,000 psi end of the pipe. Doesn't surprise that it's acting sort of like a plasma cutter! I don't have anything to offer here (except that I can't think an AR15 pistol is in any way a good idea) but maybe to look into reducing gas port pressure somehow - maybe a gas vent a la an FAL?
