11.5 SBR AR15

[JayBell]

I caught that with your last post. Maybe I wasn't clear, but I still don't understand why that would make a differnce in FA vs. SA with the same rifle.

because in full auto cycling, the disconnector holds the hammer, the disconnector is tripped by the carrier hitting the auto sear to drop the hammer,

in semi, the trigger must be released to reset the disconnector, then the hammer is dropped by the trigger pull. Your finger is generally too slow to make it happen in semi.

its a mixture of mechanics and timing, when it happens in FA mode, the timing is close enough that instead of the carrier bouncing back and closing naturally, the carrier is hit while bouncing open, closed by the descending hammer, and the round does not fire

[ungawa]

because in full auto cycling, the disconnector holds the hammer, the disconnector is tripped by the carrier hitting the auto sear to drop the hammer,

in semi, the trigger must be released to reset the disconnector, then the hammer is dropped by the trigger pull. Your finger is generally too slow to make it happen in semi.

its a mixture of mechanics and timing, when it happens in FA mode, the timing is close enough that instead of the carrier bouncing back and closing naturally, the carrier is hit while bouncing open, closed by the descending hammer, and the round does not fire

Ok, thanks, very helpful. Let me put it another way to see if I got it right.

Regardless of fire mode, once the bolt locks the carrier then continues and bounces off of the barrel extension and is driven back into place by the buffer/spring. When in FA mode, the hammer can drop before the carrier is driven back into place and the hammer can be obstructed by the carrier.

Is that right?

I need to let my hammer ride the BCG and other experiments now to see how that happens. Thanks again for the info, this may come in handy in the future.

[JayBell]

Ok, thanks, very helpful. Let me put it another way to see if I got it right.

Regardless of fire mode, once the bolt locks the carrier then continues and bounces off of the barrel extension and is driven back into place by the buffer/spring. When in FA mode, the hammer can drop before the carrier is driven back into place and the hammer can be obstructed by the carrier.

Is that right?

I need to let my hammer ride the BCG and other experiments now to see how that happens. Thanks again for the info, this may come in handy in the future.

correct. the heavy buffer helps prevent the bounce, and the 2 piece commando buffer acts like a deadblow hammer to stop it completely.

[ungawa]

OK, guess what? I got bolt (carrier) bounce now.

Today we had two light primer hits out of about 300 rounds on a Frankenupper, 11.5" barrel. This thing only jammed once previously with several thousand rounds down the pipe. She's a beater.

I strongly doubt this is a statistical inconvenience, what changes over time cause this? I replaced the buffer spring about 2 years ago for no particular reason. It is an A1 buffer and stock, not carbine.

[class2SOT]

Good posts on the gas port issue. This is common with SBRs.

One concern is to not go below 55 grains in bullet weight. The 75 and 77 grain loads work wonderfully.

[ithinkimamedic]

How are your gas rings looking? When were the last replaced, while it's likely not the exact cause it could be a contributing factor as the rings effect the friction the bolt and carrier group. At least it's a theory though it could be completely off basis.

[ungawa]

Jaybell,

Would the two-piece buffer you use slow down my ROF? With this upper I actually appreciate the higher rate of fire. I have not timed it but would guess it runs around 1000. Where would I get a buffer like yours? I have a heavy buffer but I hate the slow ROF it yields.

[JayBell]

I dotn think the ROF would change, but, who knows.

another issue is gas port erosion... since the heat on the SBR barrels tends to concentrate the most around the gas port area, over time, with a lot of fire, the gas port erodes larger, giving even more of a gas shot than normal.

the only solution to this is a new barrel, or, if its a chromoly steel barrel, weld and redrill.

however, using a A1 stock with the rifle buffer, its rare to see carrier bounce , so unless it happens often.. I'd more likely suspect a fluke...b.ut, if ya can, switch to an A2 buffer and spring.. it'll work just fine in the A1 stock tube.

check yer FCG to be sure the timing it right also.

the two piece mechanical buffer I got from DPMS, its a copy of the one Colt put in thier `10.5" commando guns, also, its for CASBINE stocks only unless you put a spacer in the A1/A1 buffer tube to make up the length difference.