300 BO again

[Howler]

I put together a BO AR last year and it wouldn't lock back. With tons of other stuff going on I never got around to fixing it. Well, today I took it back out and it still wouldn't lock back or cycle the next round. Using super loads with the suppressor on the barrel it only loaded the next rounds 2 out of 10 times. Looked as if the gas block was lined up and all. Gas rings were good, key was good, just seems not enough gas is getting back through the tube. I'm going to break it back down and measure the barrel port and block port and try to see what's going on. What are the measurements supposed to be?

[ehull20000]

Not sure if it matters on the 300 BO, but my 10.5 pistol AR required experimenting with buffer weights. I also installed a Micro MOA Govnah to help dial in the gas port diameter for suppressed and unsuppressed.

What length is your barrel?

What length is your gas system?

[Howler]

Barrel length is 16" and the gas system is pistol length. The buffer is a standard carbine buffer. The gas block is an adjustable block but when I removed it the first time I made sure it was completely open and actually added a little thread sealer to the adjuster screw just to make sure gas wasn't leaking from there. Now I've either lined it up wrong again or the port size just isn't big enough.

[Dolomite_supafly]

Is the ammunition factory or hand loads?

If factory what brand?

If hand load what recipe?

A 16" barrel with a pistol length gas system should run almost anything out there.

Measure the distance from the muzzle to the gas block.

[Howler]

This is what the barrel port looks like. Looks as if the block was pretty well lined up. I measured the distance between the barrel shoulder and the block shoulder and it was right at .042. The barrel port diameter is right at .067.

The ammo used was factory PNW Arms 125gr NBT hunting ammo. I figured even if the ammo was loaded light it should work with the suppressor on the end but it didn't.


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[Quiet Tirant]

I put together a BO AR last year and it wouldn't lock back. With tons of other stuff going on I never got around to fixing it. Well, today I took it back out and it still wouldn't lock back or cycle the next round. Using super loads with the suppressor on the barrel it only loaded the next rounds 2 out of 10 times. Looked as if the gas block was lined up and all. Gas rings were good, key was good, just seems not enough gas is getting back through the tube. I'm going to break it back down and measure the barrel port and block port and try to see what's going on. What are the measurements supposed to be?

I had the same problem when I built a 16" BO for a buddy of mine. Before you drill out the gas port or anything like that I would check the receiver and bolt carrier. Remove the lower reciever then hold the upper vertically with the muzzle pointed to the ground. Leaving the charging handle locked in the forward position pull back the bolt carrier all the way to the rear and let it go. The bolt should drop all the way forward and lock. If it stops before the bolt touches the chamber you have a bend in the reciever probably on the ejection port(which I had from torquing the barrel nut). However, if the bolt slides back and forth smoothly then you might want to consider enlarging the gas port to .125 inch(1/8inch). I hope this helps.

[KKing]

If it won't lock back with the gas block all the way open the way I see it you have a couple of options.

1) Drill out the gas port as suggested *best option

2) Cut a half coil off of your buffer spring at a time until it locks back. Not the best way to do it, but I've seen it done with success, and reliability.


Sent from my iPhone using Tapatalk Edited November 20, 2015 by KKing

[Quiet Tirant]

1/8" is too big for a 16" with pistol length gas system.

I completely agree. Missed that part about the pistol length.