.300BLK Short cycling? Overgassed?

[Caver Dave]

I've searched... Most findings are with a collections of parts/components that don't match mine or different symptoms.

Over a year ago, I purchased a (Diamondback?) .300BLK upper "4150 melonited 1/8" twist 16" barrel, pistol length gas". I needed something "easy shooting" (narrow gauge family members can't take the 12GA), decent mag capacity, with a bit more ompah than 5.56 (for larger "varmints"), 100% Supersonic use, and keeping the platform light.

I sat on it until late last year, as I wanted another lower, and purchased an assembled Anderson lower (unknown carbine buffer/spring) from a friend at Patriot Arms & Ammo (Appomattox, VA). I added a PSA NiB FA BCG & std. charging handle to the mix.

After finally getting it out few weeks back using 110gr Hornady Black, I could at most get 1-2 rounds before jamming... either not fully ejecting the spent case/not stripping & chambering the next round/both simultaneously.
Thinking it might be the mags, swapped between all I have (Troy, IMI, Magpul, & GI steel) with the same results. Pulled it all down, wiped, re-lubed, & assembled... same results.

That's when I started looking the symptoms and resolutions (gas port undersized, carbine length gas, too heavy BCG/buffer/spring, etc.), but nothing that jumped out... aside from some sources that said after a few hundred cycles, everything started running fine, attributed to "break in" (I can see all new components needing time to "slick up"). My assumption that the PO (@Flashpoint) mainly ran subsonics, so don't think there's anything wrong with the upper?

Please correct me if wrong, but:
It appears that undersized gas ports more an issue with carbine length gas than pistol length?
A FullAuto BCG is more of an issue running subsonics?
My carbine buffer/spring should be fine?
Will try different ammo, but not sure how much factory ammo would effect cycling in this case?

Any other ideas?

 

[Ikarus1]

First try different factory supersonice ammo, then I would look at the gas system. Maybe the gas block is loose or has moved. Subsonic .300blk has plenty of gas volume to move that bolt.

 

[Glade8r]

I had the same issue with my pistol .300blk. It was missing the oring on the extractor.

Sent from my LM-V350 using Tapatalk

 

[Caver Dave]

@Glade8r I saw that as as a potential, but hadn't checked for the o-ring! Will disassemble & check tonight...

 

[JRHorne]

I'd bet the BCG is the problem. Had a similar problem with a PSA BCG in my 5.56 pistol. Find a decent BCG and swap that in with everything else the same and see what happens.

 

[Caver Dave]

I'd bet the BCG is the problem. Had a similar problem with a PSA BCG in my 5.56 pistol. Find a decent BCG and swap that in with everything else the same and see what happens.

Reckon the Spikes FA BCG in my 5.56 is "decent" enough? It's put a couple 1000 downrange trouble free since building that hodge-podge...

 

[JRHorne]

Reckon the Spikes FA BCG in my 5.56 is "decent" enough? It's put a couple 1000 downrange trouble free since building that hodge-podge...

Absolutely

 

[Hammer12b]

I’ve got a .300 that is doing the same thing! It was short stroking the round. Pulling back about 1/2 an inch and not ejecting. My gas block was a little bit off and you could see the burn marks from escaping gas. Put it back into position and it now ejects the round but still doesn’t come back far enough to pick the next one up! I’ve shortened the recoil spring but haven’t had the time to shoot it to see if this fixed the problem.

 

[Slacker]

I’ve got a .300 that is doing the same thing! It was short stroking the round. Pulling back about 1/2 an inch and not ejecting. My gas block was a little bit off and you could see the burn marks from escaping gas. Put it back into position and it now ejects the round but still doesn’t come back far enough to pick the next one up! I’ve shortened the recoil spring but haven’t had the time to shoot it to see if this fixed the problem.

I fought this same battle with a Wilson combat barrel. After literally throwing money and time at it, I had a local gunsmith open the gas port to .110 and added an AGB.
Problem solved. It now runs bunny fart subs like a champ.

 

[Hammer12b]

I fought this same battle with a Wilson combat barrel. After literally throwing money and time at it, I had a local gunsmith open the gas port to .110 and added an AGB.
Problem solved. It now runs bunny fart subs like a champ.

Mine was really bad when I ran it suppressed!

 

[Caver Dave]

Pulled the PSA FA BCG down the other night... definitely has the o-ring on the extractor spring

If it warms a bit Monday, hoping to drag it back out... swapping BCGs, buffers/springs and also grabbed a box (AE?) of 150gr supers to try. If that doesn't work, will pull it down to check the gas block alignment & port diameter.

 

[JRHorne]

With everything wiped down really well (BCG, inside of the upper receiver, charged handle),dry charge it with either an empty mag or use the bolt release to hold the bolt back. Then use bolt release to allow it to close. Do that 2 or 3 times in a row. Do you smell anything? Get us some good pics of the outside of the carrier.

 

[Caver Dave]

So, after only a year , I drug the .300blk back out (stoopid busy since moving to the Commonwealth) on Sunday and gave it another try... fully prepared to start swapping parts (BCG, spring/buffer)
Basically, the only thing that'd been done to it since last post = oiled the BCG, a couple points on the charging handle/upper, buffer spring and cycled it 2-300 times.

Ran 10 rds (same Hornady Black 125gr as before) thru it without issue, then ran a 20 rd mag, followed by 10 more, and closed with a 20rd. mag dump... absolutely ZERO issues!
60 rds without a hitch... Was so happy, danced around like a 3 y/o on Christmas day!
Not sure what (BCG/upper? buffer tube/spring) was a little tight, but appears all is well?

Planning to drag it back out this weekend to get the irons sighted! (still waffling on whether the old 4th hand glass will go on or not... keeping it slim/trim/"un-intimidating" for the rest of the family)

 

[Alabamacoastie]

So, after only a year , I drug the .300blk back out (stoopid busy since moving to the Commonwealth) on Sunday and gave it another try... fully prepared to start swapping parts (BCG, spring/buffer)
Basically, the only thing that'd been done to it since last post = oiled the BCG, a couple points on the charging handle/upper, buffer spring and cycled it 2-300 times.

Ran 10 rds (same Hornady Black 125gr as before) thru it without issue, then ran a 20 rd mag, followed by 10 more, and closed with a 20rd. mag dump... absolutely ZERO issues!
60 rds without a hitch... Was so happy, danced around like a 3 y/o on Christmas day!
Not sure what (BCG/upper? buffer tube/spring) was a little tight, but appears all is well?

Planning to drag it back out this weekend to get the irons sighted! (still waffling on whether the old 4th hand glass will go on or not... keeping it slim/trim/"un-intimidating" for the rest of the family)

Im glad this issue resolved itself for you.

I built a .300bo pistol and spent forever changing out parts, but I finally have it running reliably now. The culprit was the buffer spring...

 

[Hammer12b]

My .300 did the same thing. I changed out the BCG, twice, changed the buffer, spring, gas block and tube. I was going to part it out and before I did, I changed the barrel. Ran like a sewing machine after that! I think it was getting too much gas. I should have tried an adjustable gas block . Anyway, it’s all good now!