Informed/Expert AR 15 help

[hpdrifter]

Just bought my first AR. I got a 300 BO upper for it also.

The 5.56/223 functions flawlessly. The BO doesn't cycle enough to pick up a cartridge or lock back. I've tried about 4 different brands. Remington(gasp) cycles about 1/2 of the time, the others.......nada.

When I bought it(the BO), at a gunshow, I had them change the gas block to one with a rail so I could mount a front sight.

I'm thinking the gas block isn't lined up right. It looks square to the front rail, but maybe not located on the barrel properly. The gun shop that sold it to me is saying I need to change the buffer spring because BO doesn't produce the thrust/pressure needed. I don't know, but that sounds strange. I haven't read anything about that on the web.

Anybody got something to add to this or suggestions?

[6bg6ga]

I think the gas block is probably leaking. This is a common problem and it can be fixed. Some gas blocks don't seal very well and it just possible the combination they have is a touch too loose.

[Epd230]

Is it an adjustable gas block? It may need to simply be adjusted. Pictures would help.


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[Moleman-]

Take the block off and look at the carbon staining around the gas port and gas passage on the gas block journal. If it's not centered front to back remove the rear setscrew on the gas block and install it upside down centering the screw hole over the gas port. Measure the gap between the block and the step in the barrel with a feeler gauge and then flip it back over keeping the gap the same. While the gas block is off, visually check that the gas tube hole and gas block passage are aligned. What ammo are you using, standard or subsonic, and what is the gas system length and port dia?

[Ford SD]

Just bought my first AR. I got a 300 BO upper for it also.

The 5.56/223 functions flawlessly. The BO doesn't cycle enough to pick up a cartridge or lock back. I've tried about 4 different brands. Remington(gasp) cycles about 1/2 of the time, the others.......nada.

When I bought it(the BO), at a gunshow, I had them change the gas block to one with a rail so I could mount a front sight.

I'm thinking the gas block isn't lined up right. It looks square to the front rail, but maybe not located on the barrel properly. The gun shop that sold it to me is saying I need to change the buffer spring because BO doesn't produce the thrust/pressure needed. I don't know, but that sounds strange. I haven't read anything about that on the web.

Anybody got something to add to this or suggestions?

I have only assembled app 6 uppers
1st one was a 300bo 16 inch barrel --reg buffer --carbine gas tube

the gas block has to have a gap of about .025 between the shoulder on the barrel and the gas block to allign correctly --> the hole in the barrel

99 % of the factorey ammo will work the action
the exception is some of the very short barrels 8- 10 inch that need a can to extend the pressure curve
at least that is what I have read
no cans where i live

[hpdrifter]

I think the gas block is probably leaking. This is a common problem and it can be fixed. Some gas blocks don't seal very well and it just possible the combination they have is a touch too loose.

Thanks. I'm gonna let them look at it first.

Is it an adjustable gas block? It may need to simply be adjusted. Pictures would help.


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Not an adjustable block. It's typical block with rail on top and bottom. Seems the gas tube fits rather well.

Take the block off and look at the carbon staining around the gas port and gas passage on the gas block journal. If it's not centered front to back remove the rear setscrew on the gas block and install it upside down centering the screw hole over the gas port. Measure the gap between the block and the step in the barrel with a feeler gauge and then flip it back over keeping the gap the same. While the gas block is off, visually check that the gas tube hole and gas block passage are aligned. What ammo are you using, standard or subsonic, and what is the gas system length and port dia?

Thanks for the instruction. I'm gonna let them look at it first. The block looks to fit the barrel rather well. Using regular 125 Remington, winchester, and Hornady. No subsonic yet. It is carbine length.

My main concern is them tinkering with the buffer system when that is probably not the issue......????????

[Plate plinker]

They messed with the block, then start with the block.

[dk17hmr]

Unless there is a crazy heavy spring and buffer in your gun it should work with the same setup as a 223. Your not getting enough gas for some reason. Either a leaky block or misaligned.

[hpdrifter]

They messed with the block, then start with the block.

Unless there is a crazy heavy spring and buffer in your gun it should work with the same setup as a 223. Your not getting enough gas for some reason. Either a leaky block or misaligned.

That's what I was thinking, check the gas block first.

[country gent]

On the gas block not only check the front to back but the radial alighnment of the gas port holes. Also check the gas tubes hole alighnment to the gas blocks port hole. these all need to line up for the gas flow to allow the rifle to function. Last is to check for binding of the gas tube in the key on the bolt carrier and the amount it enters into the key. Another place to look if a used upper is the gas rings on the bolt. If standard then check for wear and that the splits arnt in alighment with each other. Like rings in a engine the rings seal and the splits alighned allow bleed by. If this is a clamp on gas block and not the pinned I'm betting theydidnt get it alighned radially close enough.

[Plate plinker]

On the gas block not only check the front to back but the radial alighnment of the gas port holes. Also check the gas tubes hole alighnment to the gas blocks port hole.

EXACTLY thanks for clarifying.

[Cottonpicker]

drifter
Is it a carbine or pistol gas barrel? If it is a carbine gas system it can be problematic. Before I start breaking down a gas block on a carbine gas barrel, I would look at a reduced buffer spring. Nemo makes a reduced buffer spring especially for 300 blk carbine gas systems. I have an Colt SP1 which I set up with a retro 300 blk barrel (carbine gas) to pair with the original 223 upper. When shooting the 300 blk, I change out the buffer spring and I works perfectly. If I use the standard 223 buffer spring with the 300 blk upper it doesn't cycle properly or lock back on the last round. Generally when setting up an AR15 in 300 blk, it's best to use a pistol gas barrel!

[hpdrifter]

drifter
Is it a carbine or pistol gas barrel? If it is a carbine gas system it can be problematic. Before I start breaking down a gas block on a carbine gas barrel, I would look at a reduced buffer spring. Nemo makes a reduced buffer spring especially for 300 blk carbine gas systems. I have an Colt SP1 which I set up with a retro 300 blk barrel (carbine gas) to pair with the original 223 upper. When shooting the 300 blk, I change out the buffer spring and I works perfectly. If I use the standard 223 buffer spring with the 300 blk upper it doesn't cycle properly or lock back on the last round. Generally when setting up an AR15 in 300 blk, it's best to use a pistol gas barrel!

it's a carbine barrel. It's in the hands of the 300 BO purveyor now. I'll see what he comes up with.

Which brings me t another question........Why isn't 300 BO loaded to the same pressures???? Or close.

Yet another question....Is there any of these issues with 458 Socom?

[Texas by God]

I run carbine length gas tube and carbine buffer/spring in my .300 with no problems. I don't shoot subsonic; I've shot 110gr to 180 gr bullets and only one box of factory 120gr. I think it's the gas block, too.
Thomas

[DW475]

Haven't run into any of those issues with my 458 Socom upper.

[6bg6ga]

I prefer pistol gas length myself but all will work with the 300 BLack Out. I've built them all and they all work. Its NOT a question of the length but rather gas leaking and thus not enough to cycle the bolt and expell the round. I've loaded 300 blk with everything from the minimum loads to the maximum all all have worked in any 300 blk I have built.

[Hamish]

"The gun shop that sold it to me is saying I need to change the buffer spring because BO doesn't produce the thrust/pressure needed. I don't know, but that sounds strange. I haven't read anything about that on the web."

Whomever told you this is, to put it politely, is woefully lacking in actual knowledge on the subject.

[SNDBGGR1484]

I run 300 BLK. in both carbine and pistol gas systems using a standard 223 buffer and spring.It sounds like you are having a gas pressure problem.You can put a piece of vacuum tubing on the gas tube and blow through it to see if the gas block and gas port are lined up. If they are lined up the next question would be "What is your gas port size ?" You may need to open up the gas port.Be very careful when opening up gas port size going up in very small increments. When drilling gas port be sure to put liner in barrel so you don't drill too far into the lands and grooves.Also check the gas key on the BCG to be sure it's on tight.

[country gent]

With the ARs gas impingment system, not only is pressure of the gas important but the volume of the gas is also important. The larger bore coupled with the slightly lower powder charges and different powders may be starving the system for gas not on pressure but the volume as well. I'm betting there is a misalighnment or burr somewhere blocking passage

[hpdrifter]

Thanks guys. He said he put a BO buffer spring in it and it ran great. Will this harm my buffer/bolt when shooting 223?

When I get a few more "miles" into this rifle and a few more gizmos lined up, I will like tear into it myself.

I ain't gonna tell him he's full of beans since he has been more than willing to look at it and generally been real friendly.

Appreciate all the input and information.