300 Blackout Subsonic

[cheek9075]

OK I have been missing around with a 300BO, Ar15. I have been trying to get it to function correctly. Here is what I have so far. I have tried H110 with 203gr bullets, H4198 with 203gr bullets, and AA1680 with 203gr bullets. I have pistol length gas system, adjustable gas block opened fully, 1/8 gas hole on the barrel. I also just got a new toy, Hybrid suppressor. I have tried a 300BO buffer spring, that let the rifle cycle, but it did not have enough spring to pick up the next round and chamber it.

I thought that the suppressor would create enough pressure to allow the rifle to cycle, but no luck. If I go up any on the powder charge, the rounds go supersonic. Any ideas?? What else can I try?

 

[Knotwild]

What does it do with factory subsonics? I also have a Hybrid and shoot it on a pistol length gas system 16" barrel. No problems with factory subsonics. I think your gas hole is big enough. If I were toying with it myself, I would try to see if the gas block is centered over the hole, but that really takes a bore scope. Make sure your bolt carrier group is clean; mine gets real dirty with some powders. You could try another bolt, especially one that is broken in some and well lubed. Maybe play with seating depth. AA1680 is supposed to work, but a lot of people say H110 won't work at all. And finally, I read that people are using a H2 buffer with a pistol length barrel and gas system.

 

[jamesl]

You might try a heavier bullet and as previously stated aa1680.

 

[dellet]

Of the three powders you have listed 1680 will be the easiest to get it running. H110 will be the hardest.

If it does not cycle with 10.5-11 grains of 1680, you have another problem. Likely gas, but it could be either a leak or blockage, check for both.

H2 buffer, FA carrier should be no problem and is basically how it was designed to run. A port of .125" is over kill. Suppressed supers may Cycle too fast.

I ran those bullets around 2.000" and varied powder depending on barrel length to stay around 1000 fps.

 

[cheek9075]

I'm loading Palmetto Projectiles 203 gr bullet. Their load data shows AA1680 at 10.2 grs of powder. I loaded all the way up to 11.2gr, same result. I am using a .460 end cap on the suppressor, but it should run without the suppressor with AA1680.

 

[dellet]

Unless the bullets are undersized, always good to check cast, .309+ would be nice, it's not the load. It should run without the suppressor.

Loose or plugged gas key, plugged tube, gas block over the port.

Buffer 4.6 oz is normal, anything less than that should be running.

Depending on the spring you tried and it failed to go into battery sounds like you possibly have something hanging up mechanically. A heavy hammer spring can cause a lot of drag on the bottom of the carrier.

It would be worth it to slide the gas block back and make sure the port has a carbon ring all the way around it.

What length are you loading to?

 

[cheek9075]

Loading to 1.950. What does the carbon ring around the gas port indicate?

I have a CMJ drop in trigger. It does the same thing with a Jard trigger in a different lower. Also tried another bolt, and buffer spring out of my 223

 

[dellet]

The carbon build up around the port is from the hole in the gas block being larger than the port. for reference that port is .096". If it's not centered you will see the port being covered

That's one that was leaking do to a poor fit. you can still see the port was centered, but all the gas was not going down the tube.

How long is your barrel? I don't remember seeing that, but wouldn't be the first time I forgot something.

 

[cheek9075]

Barrell is 16 inch.

I will check the gas port tonight. I know the gas port hole is 1/8.

 

[cheek9075]

I checked the gas port, no carbon ring... the gas hole in the barrell is 1/8 inch the hole in the gas block measures 1/8. Barrell has indention for set screw on gas block. I do not see how it would miss align. Gas system is open, by the looks of the bolt carrier, I'm getting alot of gas back to the bolt. I cleaned, and lubed everything.
I'm going to stop at the gun shop and see if the have any factory rounds in subsonic and give that a try.
Shot 3 rounds over crono, ranged from 1005 to 1040 fps, load data shows that should create 25,000cup. I don't know, nothing ever seems to be easy with an AR for me.

 

[rebel]

I checked the gas port, no carbon ring... the gas hole in the barrell is 1/8 inch the hole in the gas block measures 1/8. Barrell has indention for set screw on gas block. I do not see how it would miss align. Gas system is open, by the looks of the bolt carrier, I'm getting alot of gas back to the bolt. I cleaned, and lubed everything.
I'm going to stop at the gun shop and see if the have any factory rounds in subsonic and give that a try.
Shot 3 rounds over crono, ranged from 1005 to 1040 fps, load data shows that should create 25,000cup. I don't know, nothing ever seems to be easy with an AR for me.

How's the fit gas block/barrel? Slide on easy, tap on?

 

[dellet]

Make sure you check the bullet diameter and it might be worth slugging the barrel to check bore diameter if the bullets are .310 like they should be. If you can recover a bullet, check if it is black due to gas is blowing by it.

If this is the first sub load you have tried with this rifle, it might be worth trying a different bullet. Preferably a jacketed one. Maybe a 208 Amax or even a 190 grain if you have something available.

 

[cheek9075]

Fit no gas block is okay.... between tap on and slide, I can put it on by hand, but have to use force.

Rifle shoots 110 vmax with 19.2gr in a 1/2 inch at 100 yards, have crono that load.

Doesn't look like there is a gas leak, I am getting gas back I know with suppressor. Tried my buddy Omega with 308 cap, rifle does function with that. I would like it function with out suppressor though.

I will Messure bullet dia. Tomorrow and post. Also going to look for some other bullets at gun shop, I will see what they have to test with.

 

[mytwo60]

If you do all three below it will cycle:

1/ Use a heavier buffer
2/ Use a heavier bullet (at least 208grn)
3/ AA1680

 

[cheek9075]

Bullets measures. 310-.3105. going to try lighter buffer spring and heavier buffer tomorrow. Only thing I know left to try.

 

[Knotwild]

I forgot to mention that you might check the gas rings on the bolt.

 

[cheek9075]

Finally got this rifle to operate. After trying everything I called a buddy and discussed what was happening. He said try another bolt, the b&c seemed to have a ruff spot in it. Changed bolt, test fired with and without suppressor, operating now!!

Thanks for advise and replies.