A1680 or CFE BLK for more gas in Subsonic 300 BLK?

[GyroF-16]

BLUF / TLDR: do you recommend A1680 or CFE BLK (or something else?) to:
(In order of priority)
- drive a 190 gr subsonic Hornady SUB-X (essential)
- generate enough gas to reliably run a suppressed 8” SBR (also essential)
- be as quiet as possible (desirable)

Long version:

I finally got my suppressor for 300 BLK (SF 762-MINI2), and am testing subsonic ammo.

I’m using an 8” Aero upper.

I can get 220 gr subs to cycle and lock back with the same buffer/spring setup I use for supers. This was a pleasant surprise.
And 200 gr subs will function with lighter buffer/spring.
But my “primary” desired load - Hornady 190 gr SUB-X will cycle, but not consistently lock back with even a light carbine buffer and spring equivalent (I have a LAW folder installed, so reduced the buffer weight to generate the same reciprocating mass as a standard BCG and carbine buffer).
I’m a bit puzzled, as I didn’t expect to be undergassed when suppressed (although Surefire emphasizes their low back pressure).
So, now I can:
1. Lower reciprocating mass by getting a low-mass BCG (can save 3.4-3.7 oz) for $150-250. This is the most expensive solution, but would certainly address the issue. And a titanium BCG would be cool to look at.
2. Lower buffer spring resistance with a weaker spring (already ordered from Sprinco for $30 shipped)
3. Increase gas pressure by loading a “hotter” 190 gr SUB-X load

I’m digging into pursuing the 3rd option. The Hornady factory load is spec’d for 1050 fps out of a 16” barrel. I’m certainly getting a fair bit less than that out of 8”, so I figure if I use Hornady load data spec’d for 1100-1150 from a 16” barrel, I will likely still be subsonic out of 8”. And (most importantly), generate more gas to run the gun. I plan to use an iterative process to avoid the supersonic “crack” while keeping the charge as high a possible for gas to run the gun.

I’m going to order the Hornady projectiles tonight.

Will A1680 or CFE BLK best meet my needs?

@Trigger

[TOTS]

I was getting reliability from a 9 inch barrel with IMR 4198 under Hornady 210s. Never Chrono’d any of it, though.

[Trigger]

Gyro,

“Will AA1680 or CFE BLK work?”

Yes. With a subsonic load, you will drive the bullet supersonic before you over-pressure the case/primer/powder combination. I would work with a chronograph, and ideally an adjustable gas block. From memory 10.8gr AA1680 with a 220 sierra is subsonic around 1050. With the 190 Hornady, I would increase powder charge until velocity was around 1020-1050fps. Then if you load a mag with one round, and it does not lock back, then increase the gas volume on the adjustable gas block.

I’m less familiar with playing with bolt carrier/buffer/spring rates to achieve the same effect. But experiment and see what works. Maybe find a known good combination with that bullet and powder on silencertalk or other forums. My 9” AAC upper (10 years old) seems to work well for 220 subs and 125 supers, but I have not tried the 190s. Same with my MCX 8” upper.

I’m not sure if one powder or the other will generate more gas. Report back with your results!

[GyroF-16]

Thanks, @Trigger

If only I had access to a chronograph…
Thanks for the idea of checking silencertalk, I hadn’t thought of that. Was looking at a 300BLK forum, but didn’t see anything there related to my challenge.

I’d contemplated an adjustable gas block, but my thinking was that if I’m undergassed (for 190 subs) right now, I wouldn’t get any more through the barrel’s gas port even with an adjustable block wide open.
Am I misunderstanding how an adjustable block works?

Gyro,

“Will AA1680 or CFE BLK work?”

Yes. With a subsonic load, you will drive the bullet supersonic before you over-pressure the case/primer/powder combination. I would work with a chronograph, and ideally an adjustable gas block. From memory 10.8gr AA1680 with a 220 sierra is subsonic around 1050. With the 190 Hornady, I would increase powder charge until velocity was around 1020-1050fps. Then if you load a mag with one round, and it does not lock back, then increase the gas volume on the adjustable gas block.

I’m less familiar with playing with bolt carrier/buffer/spring rates to achieve the same effect. But experiment and see what works. Maybe find a known good combination with that bullet and powder on silencertalk or other forums. My 9” AAC upper (10 years old) seems to work well for 220 subs and 125 supers, but I have not tried the 190s. Same with my MCX 8” upper.

I’m not sure if one powder or the other will generate more gas. Report back with your results!

[alamo5000]

I bought a few thousand 190 grain bullets a few years ago and between 1680 and CFE BLK, both work just fine, but the clear winner is Shooter's World Blackout. With that particular grain weight of bullet it was a better performer than either of the other two.

Edit: To add more context and info, I am shooting a 9" SBR with I think an H3 buffer. Between supers and subs I didn't have to alter not a single thing. Also Blackout powder has much less port pop when compared to either of the other two. In other words it's quieter and an overall better experience.

[Trigger]

Thanks, @Trigger

If only I had access to a chronograph…
Thanks for the idea of checking silencertalk, I hadn’t thought of that. Was looking at a 300BLK forum, but didn’t see anything there related to my challenge.

I’d contemplated an adjustable gas block, but my thinking was that if I’m undergassed (for 190 subs) right now, I wouldn’t get any more through the barrel’s gas port even with an adjustable block wide open.
Am I misunderstanding how an adjustable block works?

Gyro, no you are correct. If the adjustment on an adjustable gas block is wide open, then the barrel gas port size is too small. I think you should be able to get either powder to work. Lemme do some research.

[Trigger]

Found this. As with all reloading data on the internet, treat with a good bit of skepticism.

https://www.snipershide.com/shooting...a.53304/page-3

Post 105 and 107 have some starting points.

[GyroF-16]

Thanks!
Funny, I just found that data in post #105 in a review of 190 SUB-X at opticsplanet.
Looks like he was seeing a drop of about 100 fps from Hornady’s published data across the board when using a 9” barrel.
That leads me to believe that I’d be able to stay subsonic when using charges approaching the max with an 8” barrel.

For the benefit of other readers, here’s the guy’s data with my assessment comparing it to Hornady predicted velocity:

300 AAC Blk, 9” Ballistics Advantage Barrel, 1:8 Twist
Hornady 190 grain Sub-X
FC12 cases
CCI 400
A1680
2.070” COAL

Charge Low High Ave ES Sd Hornady Data predicts:
12.8 1082 1125 1107 43.79 16.85 1185 fps (-78)
12.5 1022 1067 1054 44.55 18.05 1170 fps (-116)
12.2 887.3 974.1 948.1 86.83 32.17
11.9 959.8 999.4 980 39.55 14.14 1090 fps (-110)


Now I just need to find the Hornady 190 gr SUB-X in stock somewhere.
A place called coastal firearms shop seems to have them in stock, but seems to require a minimum purchase of $180, which seems excessive, and makes me a bit skeptical.

Found this. As with all reloading data on the internet, treat with a good bit of skepticism.

https://www.snipershide.com/shooting...a.53304/page-3

Post 105 and 107 have some starting points.

[Squib308]

My 300 blackout rifle has a BCM barrel with carbine gas. It has been tricky to achieve reliability with suppressed subsonics. Even factory hornady 208gr ammo cannot cycle the action. My handloads use AA1680 with a 208 gr projectile and have zero issues cycling. In fact even with Chrono as low as like 800 ft/s we’re still cycling fine. AA1680 has been hugely helpful for my purposes. In fairness I haven’t played with CFE black at all because haven’t needed it.

[GyroF-16]

After multiple iterations, this has worked well for me.
It’s functional with an 8” Aero upper with a Geissele 42 spring and an H3 (5.38 oz) buffer and a LAW folder (2.17 oz BCG extension) used with a SF 762MINI suppressor.


190 SUB-X
12.1 gr AA 1680
2.070 COAL
1035 fps from 8” barrel suppressed (at 6,800’ MSL)