Interesting variance in gas port size

[USK308]

Semi long post, but good info to share in my opinion)

So I recently finished building my 300Blackout SBR (10.5" Faxon Gunner-profile barrel with pistol length gas). First time at the range to dial in the adjustable gas block (Aero Precision), using 100gr A-Max Hornady (had a few boxes I bought for good price), I started out with the gas block wide open. One round at a time. Thought I had it dialed in, but the rifle would only shoot one maybe two rounds before I would not load another round. Went through this cycle 3 times. Hmmm...strange, weird!

Headed home to ponder.

A week later, I decided I would go to the range and try some 208gr V-Max Hornady (again, what I have as I got many rounds at a good price).

Same process, starting out wide-open, only loading one round in the mag until I got the gas block dialed in. Never more than 2 rounds would be fired before failure to load.

Same situation, another three cycles of the process. Very very odd. Headed back home for more pondering.

I have a couple friends with Aero Precision AGB's that work just fine so I was baffled (I had this from a previous build where I changed my path at the last minute). Decided to buy a Superlative Arms AGB, since I have had great luck with them. I also decided to buy another barrel, just in case I couldn't get the Faxon barrel to work. After a crap-ton of research, I had narrowed down to either a Criterion CORE 10.5" 300BO barrel or a Rosco Manufacturing Bloodline 10.5" 300BO barrel. Due to availability, the Rosco 10.5 Bloodline was the faster path (and Rosco makes the Mk18 300BO barrels, so an easy decision).

While I waited for the Sup Arms AGB to come in, I took the rifle apart and removed the Aero Precision AGB. Decided to measure the gas port size (see picture below) of the Faxon barrel. The Rosco Manufacturing barrel came in today and I measured the gas port size for that barrel (see picture below). I have installed the Superlative Arms AGB on the Faxon barrel but have not yet been to the range yet. Given the gas port size of the Faxon, I don't hold much hope...but have the Rosco Barrel basically ready to install and get the SBR up and running.

Faxon Gunner profile 10.5" pistol length gas: 0.1000" gas port size
Rosco Manufacturing Bloodline 10.5" pistol length gas: 0.085"gas port size

When I can get to the range, I will try again (using the 208gr V-Max & 100gr A-Max I have, plus some 150gr Federal American Eagle ammo). If the Faxon continues to exhibit a severe over-gas, I'll likely not waste much time on it and just put the Rosco Bloodline 10.5" barrel on and go from there.

What bothers me about the situation more than anything is that barrels for small frame AR's are usually much more consistent and I don't expect this variance in gas port size. Large frame AR barrels...yeah, very inconsistent and frequently I have found that drilling out the gas port is needed to get the rifle dialed in.

 

[1911hombre]

When you say you close the gas block until you think you're "dialed in", what are you considering "dialed in"? 4:00 ejection in a nice pile? Will this thing run with an open gas block?

 

[USK308]

When you say you close the gas block until you think you're "dialed in", what are you considering "dialed in"? 4:00 ejection in a nice pile? Will this thing run with an open gas block?

Following the procedures from Aero Precision, dialing the AGB in until I can squeeze off rounds consistently, then back the AGB off 1-click and the round ejecting at 3:00-3:30. I followed the prescribed process 3x for each session at the range.

The rounds were ejecting at ~3:00, sometimes at 2:45...which is where my brain started thinking (dangerous, I know! LOL) the gas port size was too large.

I also noted during disassembly that the AP AGB was a bit looser on the barrel when compared to the Superlative Arms AGB when I put that ON the barrel. Maybe I just have a bad quality AP AGB. Who knows.

 

[msgriff]

Following the procedures from Aero Precision, dialing the AGB in until I can squeeze off rounds consistently, then back the AGB off 1-click and the round ejecting at 3:00-3:30. I followed the prescribed process 3x for each session at the range.

The rounds were ejecting at ~3:00, sometimes at 2:45...which is where my brain started thinking (dangerous, I know! LOL) the gas port size was too large.

I also noted during disassembly that the AP AGB was a bit looser on the barrel when compared to the Superlative Arms AGB when I put that ON the barrel. Maybe I just have a bad quality AP AGB. Who knows.

Unless Aero has changed their tuning procedure, you start wide open, single load from a mag, adjust down (CW) 2 clicks at a time until you no longer get "last round bolt hold open" (LRBHO). Then open (CCW) 1 click to confirm proper LRBHO function for a few rounds. Finally, you open (CCW) 2 more clicks for varying conditions/ammo.

 

[USK308]

Unless Aero has changed their tuning procedure, you start wide open, single load from a mag, adjust down (CW) 2 clicks at a time until you no longer get "last round bolt hold open" (LRBHO). Then open (CCW) 1 click to confirm the LRBHO for a few rounds. Finally, you open (CCW) 2 more clicks for varying conditions/ammo.

Yeah, that's the basic process I followed. I did the 2 CCW clicks, but could still only get 1, maybe 2 rounds off before it would not load correctly (likely due to oversized gas port on the Faxon barrel).

We'll see how things go when I next get to the range.

 

[CRPCustomMachine]

Buffer and buffer spring rate adjustment goes a long way to improving the over gassed condition that can manifest itself on shorter barrel and 300BLK calibered AR's.

SA gas blocks work well, use them often on customer guns.

Sometimes once the ejection pattern is between 3 to 4 o'clock through GB adjustment but the gun will only cycle 2 or 3 rounds, a slightly stiffer buffer spring helps to correct the continued feed issue. For spring issues, I tend to use the JP silent capture system but a simple Sprinco Orange usually does the job.

If it is over gassed a heavier buffer also can make the difference in cartridge feeding and recoil felt.

Once dialed in - a click or two on the AGB is usually all it takes with ammo differences. Hope this helps you get it working like it should, regards

 

[S197]

Your first problem was running an Aero AGB. When tuning an AGB, start with it completely closed and open it 1 click at a time until the bolt cycles, then adjust case ejection

 

[USK308]

Your first problem was running an Aero AGB. When tuning an AGB, start with it completely closed and open it 1 click at a time until the bolt cycles, then adjust case ejection

The Aero AGB is no longer on the barrel. Bought a Superlative Arms AGB and will be going to the range tomorrow afternoon. If I still cannot get the rifle dialed in, due to gas port being too large, I will remove the Faxon barrel and put the Rosco barrel on along with the Sup Arms AGB and go from there.

 

[FredHammer]

The Aero AGB is no longer on the barrel. Bought a Superlative Arms AGB and will be going to the range tomorrow afternoon. If I still cannot get the rifle dialed in, due to gas port being too large, I will remove the Faxon barrel and put the Rosco barrel on along with the Sup Arms AGB and go from there.

That is a logical step process, and a process/components I used when I drilled holes too big. The SA GB bleed off saved the barrel from the dumpster of shame. Lol.

 

[USK308]

That is a logical step process, and a process/components I used when I drilled holes too big. The SA GB bleed off saved the barrel from the dumpster of shame. Lol.

Having an engineering mindset (thanks Dad [lol]), I try to break down issues into logical steps. The more builds I do, the more comfortable I am in my troubleshooting process. Still have a ton to learn, but have solid confidence I can put together any small or large frame AR and get the rifle working.

 

[hlee]

I put an adj gas block on my 300 blackout. It ran super and subs, both suppressed and unsuppressed without issue. There was one brand of subs that wouldn’t lock back unsuppressed (but they were just bought for testing). I took the adj gas block off (to use on another rifle) and replaced it with a low pro “normal” gas block. It still runs supers and subs, suppressed or unsuppressed, without issue. For my money, tuning a 300BO via a gas block, is unnecessary effort.

That said, if you cannot find a setting on an adj gas block that allows your rifle to function, it is likely not the gas block.

 

[USK308]

My gut instinct tells me I have a couple of issues:
1) Aero AGB isn’t a tight a fit on the barrel as compared to the fit of the Sup Arms AGB.
2) Gas port diameter on current barrel (Faxon 10.5” gunner w/pistol length gas) is too large

Appreciate your insight and WILL catalog that info for future projects!

 

[FredHammer]

I put an adj gas block on my 300 blackout. It ran super and subs, both suppressed and unsuppressed without issue. There was one brand of subs that wouldn’t lock back unsuppressed (but they were just bought for testing). I took the adj gas block off (to use on another rifle) and replaced it with a low pro “normal” gas block. It still runs supers and subs, suppressed or unsuppressed, without issue. For my money, tuning a 300BO via a gas block, is unnecessary effort.

That said, if you cannot find a setting on an adj gas block that allows your rifle to function, it is likely not the gas block.

Exactly. Trying to use a restrictive only style GB, like the Aero, to tame a too large gas port just don't work. I know, I've seen me try it!

 

[USK308]

***UPDATE***
Went to the range on Friday with the new Superlative Arms AGB on the 10.5" Faxon barrel. Tried 110gr V-Max, 147gr Federal American Eagle, 190gr Sub-X and 208gr A-Max. All still failed in form or another at gas wide open (18 clicks). Went through a series of tests down to 15 clicks from closed and up to 23 clicks from closed. Same behavior regardless. Opted to go home and put the Rosco Mfg 10.5 Bloodline barrel on, then retest.

Saturday (Yesterda), removed Faxon barrel (10.5 Gunner profile, 0.1000" gas port diameter) and new Superlative Arms AGB. Installed the Rosco Mfg 10.5 Bloodline barrel and the Superlative Arms AGB. Set the gas to 18 clicks from closed and reinstalled the handguard.

Just got home from the range:

Tested using the normal 1 round to verify lock-back for 110gr, 147gr, 190gr Sub-X and 208gr. With gas port @ 18-clicks from closed, no issues on 110, 147 & 190. Unfortunately, the 208gr A-Max is still not ejecting round.

Loaded up 3 rounds of 110gr. V-Max...rifle cycled perfectly.
Loaded up 3 rounds of 147gr. Federal American Eagle...rifle cycled perfectly.
Loaded up 3 rounds of 190gr. Sub-X...rifle cycled perfectly.
Loaded up 3 rounds of 208gr. A-Max...rifle failed to eject the first round.

Since my SBR will primarily run suppressed (once the can is out of ATF "jail"...probably 30-45 days yet), I will hold off on any more testing since I know I can load up the 110gr and 147gr and the rifle will work just fine. Once I have the can (OCL Hydrogen L 7.62), I will spend more time dialing in the rifle (including removing 1 tungsten weight in lieu of 1 steel weight to make my JP SCS an H1 instead of the current H2).

All in all, the Rosco Mfg barrel actually felt better and didn't have as much recoil...so that was good $ spent.

 

[Green461]

I fought same thing with a faxon barrel. Discovered the gas block was machined in the wrong spot, set too far forward, so the standard length gas tube was not into the bolt where is should be, but about 3/16-1/4 inch out too far. I got a custom longer gas tube, went back to test but had other issues with the barrel, breach was machined wrong, so I just returned it.. nothing but problems.

 

[JS8588]

I fought same thing with a faxon barrel. Discovered the gas block was machined in the wrong spot, set too far forward, so the standard length gas tube was not into the bolt where is should be, but about 3/16-1/4 inch out too far. I got a custom longer gas tube, went back to test but had other issues with the barrel, breach was machined wrong, so I just returned it.. nothing but problems.

Friends don't let friends buy Faxon or Aero Precision/BA (their damned chf Hanson profile barrels look very interesting, though)

 

[Ape_Factory]

The A-Max rounds don't produce a lot of gas/pressure and I always had cycling issues with them and nothing else. This is with the same Faxon 10.5" barrel. Actually had pretty good results with that barrel TBH. Never measured the gas port on it though. Ran really well and was accurate but didn't function with the Hornady 190grain Sub-X bullets. Never could get those to work with the Faxon or a Rainier Arms barrel and got bullet setback. They work like butter with the Sig Spear LT.