My lightweight 300 blackout build requires some input

[Elkins45]

I don't think I saw where you told us what barrel you used with this build. I'm assuming either a pencil barrel or M4 configuration? Or is it something exotic I've never heard of?

BTW I installed a PSA parts kit in mine and put the upper from my old A1 configuration CAR-15 on it. It ran 50 rounds flawlessly, but I will confess I found it odd for an AR to have a noticeable recoil. It took me a second to acclimate to the gun jumping around a bit. I didn't weight it but I would be surprised if mine is over six pounds even with all conventional parts.

Thanks for letting me know these exist.

 

[R.W.Dale]

I honestly don't know who made my barrel. It was an ar15.com EE impulse buy for cheap because I liked the profile. If it shoots good I will send it to ADCO with instructions to make it as light as is safe.

 

[R.W.Dale]

UPDATE: I finally got the rifle to the range, it seems to shoot quite well however it will not run.

Amazingly enough it's UNDERgassed! Yes with an aluminum bolt carrier and essentially no buffer the darn thing is short stroking with the syrac gas block maxed out on flow. This is with gemtech 147g supersonic ammo and a BCG slathered in ATF

I think my next step would be to ditch the flat wire extra power CS spring in favor of a standard carbine spring.

It's close to running, it will lock the bolt back single loading just barely, but it just won't quite pick up that next round on a loaded mag.

 

[HankC]

Barrel length, gas system and port size are also critical. What do you have?

 

[R.W.Dale]

I don't have the port size. Carbine gas 16" bbl

 

[HankC]

16" Carbine gas system would be .125" port size typically but some vendors may have smaller port and may experience issue especially if you run subsonic. Below is a link on AR gas systems and port sizes. My 16" Carbine has .125" port.
http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

 

[Another_Kelley]

UPDATE: I finally got the rifle to the range, it seems to shoot quite well however it will not run.

Amazingly enough it's UNDERgassed! Yes with an aluminum bolt carrier and essentially no buffer the darn thing is short stroking with the syrac gas block maxed out on flow. This is with gemtech 147g supersonic ammo and a BCG slathered in ATF

I think my next step would be to ditch the flat wire extra power CS spring in favor of a standard carbine spring.

It's close to running, it will lock the bolt back single loading just barely, but it just won't quite pick up that next round on a loaded mag.

If the carbine spring doesn't turn out to alleviate the cycling issue, I'd pop that gas block off and check to see if you have a nice round carbon ring around the gas port - indicating good alignment. The gas block journal on your barrel could be the culprit.

 

[R.W.Dale]

If the carbine spring doesn't turn out to alleviate the cycling issue, I'd pop that gas block off and check to see if you have a nice round carbon ring around the gas port - indicating good alignment. The gas block journal on your barrel could be the culprit.

I think you are on the right track.

If a standard spring won't make it run I will pull the block, check for alignment and measure the port size.

I still can't believe it's undergassed, I have like NO bolt and buffer mass.

 

[R.W.Dale]

I've neglected this thread for awhile but I do have some new findings.

Even with essentially NO buffer and an aluminum bolt carrier a supersonic carbine gas 300blk is not overgassed.

As mentioned above swapping to a standard buffer spring was the key. The rifle will now run, but only with the adjustable gas block essentially fully open on a liberally oiled BCG

Knowing what I know now the high dollar gas block was unnecessary for a low mass BCG 300 blackout