300 Blackout AR Malfunctions

[darkfader]

I’m hoping you guys can help me out with this damn thing. I’ve never been able to get it function properly and I’d really like to have it working.

300BO
AERO upper and lower and aero mil-spec internals
8.5” Ballistic Advantage barrel w/ pistol length gas
Standard carbine buffer and spring
Aero adjustable gas block

Barrel has been dimpled for the gas block and the carbon fouling verifies good placement. Block and tube flow air freely when off the rifle as well.

I’m interested in running some Hornady Black 208gr AMAX rounds with a Dead Air Sandman S.

Or any damn round really.

At every click of the AGB, there is no lock back on an empty mag and no chambering of the next round in the mag. Doesn’t matter if Pmag or Okay Industries steel mag or Duramag. Doesn’t matter, won’t work.

I’d be grateful for any insights on what I can do to improve this thing other than disassembling and adding to the parts bin. I have a ton of different 300 ammo, can’t get subs or supers to work right.

Aero customer service suggested it was overgassed a year ago or so, but I can choke the AGB down to where there’s no ejection at all so I just don’t think that’s the problem. If there’s a comprehensive thread somewhere about 300 builds I’d love a link. Thanks for any help. I’m about to give up on it.

 

[USK308]

What is the weight of your buffer? Standard weight buffer isn't going to work with your setup (to my knowledge).

Are you starting out with the AGB wide open, shoot 1 round to check for lock-back? FWIW, I tried an Aero Precision AGB first but it would NOT work for $h!t...so I put a spare SupArms AGB on and after a little work to dial in....boom, I was good to go.

When I was working to dial-in my 300BLK SBR (10.5" barrel), the Hornady 190 Sub-X and the 208 A-Max were just GARBAGE. The 190 Sub-X was only marginally better than the 208 A-Max, where the 208gr A-Max never would lock back.

I finally found that 200gr S&B, 200gr Winchester Open Tip or 220gr Fiocchi Range Dynamics were the rounds that worked for me with my Otter Creek Labs Hydrogen L 7.62 suppressor. I'm running a JP H2 buffer (JPSCS15-H2) and a Superlative Arms AGB. My AGB is set at 19 clicks and the rifle runs like a dream (now) with the aforementioned ammo!

 

[darkfader]

Thanks a lot for that. I figured Hornady was G2G. I have some Fiocchi I think, I’ll check. I’ll see about a superlative arms AGB and play with buffer weights and see what happens. Thank you very much USK308!

 

[USK308]

Thanks a lot for that. I figured Hornady was G2G. I have some Fiocchi I think, I’ll check. I’ll see about a superlative arms AGB and play with buffer weights and see what happens. Thank you very much USK308!

While I have found Hornady ammo to be pretty good in other calibers in the past, there have been a fair number of people AND reports of Hornady ammo issues. Hopefully you can find some ammo that does work for ya and get that gun running smooth as butter!

 

[hlee]

Hornady black doesn’t lock back on empty in my 300 BO, but it runs otherwise. Can you lock the bolt back on an empty mag just by pulling the charging handle? A buffer that is too long, or an extension that is too short could give a false short stroke symptom. And, over gassing can cause the bolt to outrun the magazine.

I’m pretty sure S&B loads locked back in my gun.

But, I worked up a sub load using A1680 and 208 AMAX bullets that runs and called it a day. Well, not exactly. I also made a load with 150 gr round nose as well as 125 gr bthp (both super sonic). Then, I called it a day.

 

[Sterlingb06]

I’m hoping you guys can help me out with this damn thing. I’ve never been able to get it function properly and I’d really like to have it working.

300BO
AERO upper and lower and aero mil-spec internals
8.5” Ballistic Advantage barrel w/ pistol length gas
Standard carbine buffer and spring
Aero adjustable gas block

Barrel has been dimpled for the gas block and the carbon fouling verifies good placement. Block and tube flow air freely when off the rifle as well.

I’m interested in running some Hornady Black 208gr AMAX rounds with a Dead Air Sandman S.

Or any damn round really.

At every click of the AGB, there is no lock back on an empty mag and no chambering of the next round in the mag. Doesn’t matter if Pmag or Okay Industries steel mag or Duramag. Doesn’t matter, won’t work.

I’d be grateful for any insights on what I can do to improve this thing other than disassembling and adding to the parts bin. I have a ton of different 300 ammo, can’t get subs or supers to work right.

Aero customer service suggested it was overgassed a year ago or so, but I can choke the AGB down to where there’s no ejection at all so I just don’t think that’s the problem. If there’s a comprehensive thread somewhere about 300 builds I’d love a link. Thanks for any help. I’m about to give up on it.

Gas block alignment is still problematic here even with the check on dimpling and fouling. Twist it, untwist it, twist it again to ensure maximum setting. A standard carbine buffer could possibly be a tad on the heavy side for your configuration going with a subs trying a lighter carbine buffer might prove beneficial. Adjusting the elevation with different supersonic or subsonic values could also tell if your rifle is choosy about ammo. Finally, ensure that you’ve measured the gas port size twice. Sometimes such barrels are shorter with pistol-length gas systems require adjustment to generate enough gas.

 

[thedude824]

If you have the ability I would scope into the barrel and ensure the gas ports are completely lined up. I have a nearly identical build as a trunk gun.
Does it lock back the bolt using supers, fully open block, no suppressor?
What specific configuration does it fail to lock back the bolt?
After learning that I could adjust the gas to make the rifle feel like a 22 with the suppressor on… I quickly found out that carbon fouling would start causing FTF at around 3 mags. That’s with supers. I eventually just went to a fixed block. It requires a suppressor to function with subs (non-adjustable block so fully open).
I understand there is a lot of disagreement on this, but this was the first incident that started me on my path to piston systems. No worry about carbon fouling causing failures after 3 mags, 10 mags, or even 30 mags. I’m likely moving to a piston 300blk out shortly. Likely the SCAR as I LOVE my SCAR 17 SBR with HUXWRX Flow. It’s a completely different experience.

 

[wvfarrier]

I’m hoping you guys can help me out with this damn thing. I’ve never been able to get it function properly and I’d really like to have it working.

300BO
AERO upper and lower and aero mil-spec internals
8.5” Ballistic Advantage barrel w/ pistol length gas
Standard carbine buffer and spring
Aero adjustable gas block

Barrel has been dimpled for the gas block and the carbon fouling verifies good placement. Block and tube flow air freely when off the rifle as well.

I’m interested in running some Hornady Black 208gr AMAX rounds with a Dead Air Sandman S.

Or any damn round really.

At every click of the AGB, there is no lock back on an empty mag and no chambering of the next round in the mag. Doesn’t matter if Pmag or Okay Industries steel mag or Duramag. Doesn’t matter, won’t work.

I’d be grateful for any insights on what I can do to improve this thing other than disassembling and adding to the parts bin. I have a ton of different 300 ammo, can’t get subs or supers to work right.

Aero customer service suggested it was overgassed a year ago or so, but I can choke the AGB down to where there’s no ejection at all so I just don’t think that’s the problem. If there’s a comprehensive thread somewhere about 300 builds I’d love a link. Thanks for any help. I’m about to give up on it.

It sounds like your buffer or spring is too heavy. Did you, perhaps, install a rifle buffer spring? You could also open the gas port a smidge and see if that fixes it

 

[Deepwoods]

I have a few different 300 uppers in 8-10" barrel length, and while I am hardly an expert (there are so many variables), here are a few thoughts...

I have H and even H2 buffers in some. There is some squirreliness with some subsonic ammo, and I'm planning a trip to see if carbine and H buffers help.

If it is allegedly overpassed, a standard Carbine weight and spring shouldn't be an issue...espcially with that suppressor; arent Sandman's higher back pressure?

In one instance, I also found that going to a lighter buffer weight significantly increased port pop--very noticeable with suppressed subs. I didn't love having to use that fix.

If it's actually not overpassed, then fixing that will obviously help. I had a V7 barrel that was very finicky and I had D. Wilson drill the gas port to .125 which is the largest their drop down menu has, and then let the AGP do the rest. Service was good.

(side note for thedude824, that V7 had a Adams Arms piston--this was a DIY) and I was told by AA that the piston setup needs more initial umph to get the piston moving. Thats why I opened the gas port. While it worked, that V7 went to a straight pull AR upper and Ive decided to leave the piston builds for OEM buys. Generally, I do like piston + flow thru suppressors)

A Sprinco Yellow recoil spring is their reduced power spring; that is an easy/ cheap option to try. Especially if you don't plan on supers often.

I have found that 208 AMAX to be the least reliable in my setups. The most reliable subsonic I have is Federal AE 220. Sig 220 has worked too. Those rounds USK308 listed are also worth trying.

 

[bfoosh006]

FWIW... KAK sells a LtWt buffer kit
https://kakindustry.com/lightweight-carbine-buffer-set

And Tubbs sells a LtWt recoil spring
https://www.davidtubb.com/ar15-buffer-spring-stainless

 

[longshot2000]

what is the twist on the barrel?

We have found .300 BO with very short barrels and pistol gas and suppressors to be very hard to tune to get the right mix of everything working. I tend to like Faxon barrels for DIY builds, but only in .300 BO, since you are lobbing the bullet out and not looking for match accuracy. Anytime someone mentions BA barrels, I wonder. The two factory guns that just get it right, in my opinion, are Daniel Defense and Q's Honey Badger. DD does not sell its .300 BO barrel, but their rifle is awesome, and just works. Same with Honey Badger.

Now, Q has had years of experience with .300 BO, and their Honey Badger is running 7" 1:5 twist barrel with an adjustable gas block on pistol length gas. We cannot really recreate what they use for a BCG and buffer, which is the other factors, but we can with DD, so why not use what the gun manufacturer's use? Both guns come ready for suppression, and Q's barrel is sold with the gas block. DD is also running a 7" barrel, but in 1:7 twist. Q runs a proprietary BCG and buffer. DD runs a standard M16 BCG and the Maxim buffer and tube, but in my experience if you can run the Maxim buffer and tube, you can substitute a standard carbine tube and either an H2 or H3 buffer. I do not know the gas port diameter on either DD or Q, but that would be the next thing to look at, when you want to know what works. They are probably a little different, as they have a different twist, and Q is relying upon variable gas flow and DD is fixed.

Let's face it, there is just not the level of trial and error and correction on the .300 BO that has occurred over decades with the 5.56mm round, and even with the short barreled 5.56, it took years of back and forth to get the Mk18 with a 10.3" carbine barrel performing well and closing down the gas port size to 0.70".

If it were me, I would start by replacing the barrel, and stay away from these crazy fast spin rates until you perfect the shot. I also would bump-up the ammo to at least 200 grain.

Q's Honey Badger barrel

Not and advertisement, but for reference.

 

[darkfader]

Thanks a lot guys.

I can answer a couple of the questions from above.

First, the bolt definitely locks back manually with a pull on the CH. No doubt all the parts are the proper size at least.

As far as lock back with supers, I really don’t recall but I’m pretty sure it would be intermittent lock back, but definitely no stripping the next round off if I had more than one in the mag. I first assembled this thing in 2020 and basically tabled it when I couldn’t get it to work with subs or supers, suppressed or not.

As far as the BA barrel, I know those are budget level but it still ought to work. I was never looking for match accuracy. I suppose I need some pin gauges to measure the port to be sure it’s where it should be. Exactly what should it be?

I have some buffer weights so I can play around with the buffer pretty easily. That said, I see some comments suggesting lightweight buffer and some saying H or H2. Should I be going heavier or lighter than the standard carbine buffer? I hate to waste ammo trying to get this thing dialed. Maybe that’s just part of the game.

I appreciate all help!

 

[USK308]

Thanks a lot guys.

I can answer a couple of the questions from above.

First, the bolt definitely locks back manually with a pull on the CH. No doubt all the parts are the proper size at least.

As far as lock back with supers, I really don’t recall but I’m pretty sure it would be intermittent lock back, but definitely no stripping the next round off if I had more than one in the mag. I first assembled this thing in 2020 and basically tabled it when I couldn’t get it to work with subs or supers, suppressed or not.

As far as the BA barrel, I know those are budget level but it still ought to work. I was never looking for match accuracy. I suppose I need some pin gauges to measure the port to be sure it’s where it should be. Exactly what should it be?

I have some buffer weights so I can play around with the buffer pretty easily. That said, I see some comments suggesting lightweight buffer and some saying H or H2. Should I be going heavier or lighter than the standard carbine buffer? I hate to waste ammo trying to get this thing dialed. Maybe that’s just part of the game.

I appreciate all help!

I'm no expert, but to me...it sounds like your BCG is moving too fast, thus no lockback and/or stripping next round off if more than 2 in the mag. To combat that, a heavier buffer slows the rearward movement of the BCG.

Others feel free to chime in, correct my thinking if I'm talking from pluto (lol)...

 

[darkfader]

I'm no expert, but to me...it sounds like your BCG is moving too fast, thus no lockback and/or stripping next round off if more than 2 in the mag. To combat that, a heavier buffer slows the rearward movement of the BCG.

Others feel free to chime in, correct my thinking if I'm talking from pluto (lol)...

That’s my understanding as well. I’ve got a set of tungsten weights so I can mess with the buffer that’s in it rather than buying more. Sucks it takes a whole box of shells or more to run all the way through the range on an AGB. Yes I’m a cheap ass. lol