What gas length, pistol or carbine?
In theory, it should be pistol gas, but a few carbine length ones are out there. For now let’s assume set up as “normal”. This would be how AAC did their testing and pitched the project.
Any barrel under 16” pistol gas, 16” and over carbine gas.
Setup is pistol gas, M16 carrier, H2 buffer 5oz
Carbine gas, M16 carrier, carbine buffer 3oz
By design it should function with and without a suppressor, subs and supers. This creates the first problems. If it will cycle subs without a suppressor, it’s terribly over gassed for supers and more so supers suppressed. Add to that, originally only bullets heavier than 200 grains were expected to function sub-sonic, without a suppressor.
Rule of thumb, the heavier the bullet, the faster the powder you can use. For the most part, faster is quieter. Always some exceptions. You use less powder for the same velocity, it burns a higher percentage in a shorter distance so less gas and noise out the barrel to suppress and less gas blowing back in your face.
The trade off is less gas to cycle the action.
This is where port size comes into play, that can be for later.
In some ways, it’s choose a bullet and make it work.
The fastest powder I have been able to have reliable function are Accurate #9 and VV N105 with most bullets 168 grains and up in a 8” barrel, without a lot of modifications, suppressed only. The suppressor makes a huge difference in back pressure, that aids in cycling. Those loads are about as quiet as a nail gun, would probably be fine in a 10.5”. My 10.5 is carbine gas so I’m limited to slightly slower powders like 296 or N110.
If you like working up loads, this cartridge can keep you busy.
To more directly answer you question, pick the bullet, then find the powder. A good example is Lehigh’s 174 grain brass bullet. I have not been able to get that particular bullet to work with N105. Yet a 168 Amax works fine. It’s a slick bullet, fast for bullet weight with comparable powder charge. Again slowing down the powder works.
Maker bullets, has sub sonic bullets from about 80 to 220 grains. I have successfully cycled the 110 grain bullets in an AR. It’s tricky and they serve no real purpose, maybe pest control in the yards, but it would be about like shooting tree squirrels with a 9MM.
To help choose a bullet and powder, look at Sierra’s load data. If there is data down to 1200 fps, generally you can tweak it to sub sonic in a shorter barrel. It will give an idea of what’s possible. Ignore the Trail Boss loads, they won’t budge the bolt carrier.
The 190 Hornady is a good choice, and bullets in that weight class. A good practice bullet is the 190 grain Nosler competition bullet. It can generally be found in a 1000 count box for under $300. In good times shooters pro shop has blems.
Bottom line the trade off is ease of function and using powders like LT 30, 1680, CFE BLK, they will cycle almost anything. As long as you don’t mind loud, Smokey and filthy.
It was asked about Shooters World Blackout. Compares very close to 1680 and you can use that data. Consider it a hot lot and drop back 1/2-1 grain at the top end. Burns cleaner and is quieter. Also like 1680, it’s great for supers 150 grains and over.
Hope that helps make a decision. Pick a bullet and just about any powder on the burn rate chart between N105 and reloader 10 will work for something.
