Can you load a subsconic round for your AR that won't cycle the action and will still group?
Oh, that's easy. It's finding a subsonic round that WILL cycle the action and still group that's the problem. My recipe for that is:
Buffington Handload #32- Resized Remington 300 AAC Blackout Brass
- CCI Small Rifle Magnum Primer
- Hornady 208 gr AMAX Bullet
- Vihtavouri N110 Powder 8.4 gr
- COL at SAMMI max (I forget what that is at the moment.)
That cartridge is:
- consistently subsonic
- very accurate
- feeds properly in USGI magazines (Magpul mags can be a bit problematic.)
- very quiet with very low flash and smoke
Feeding? At first, in my AR, that would cycle reliably about 75 percent of the time. After much experimentation, I ended up cutting four turns off the recoil spring, and now, it cycles reliably 100 percent of the time.
So, if you really don't want it to cycle, you could go to a heavier spring or cut back the powder a bit. (If I recall correctly, 8.0 grains was still accurate, but wouldn't cycle at all.)
A disclaimer ... AR uppers, lowers and parts may be interchangeable, but that doesn't mean they're the same. Minor variations in gas ports, spring rates, hole spacing, moon phases could produce different results. And when you're trying to use an AR to propel a 200 grain bullet at subsonic speeds, you're on the ragged edge of the original Stoner design. Your mileage may vary.
Enjoy!
Dave
P.S. I erred in an earlier post. The Remington UMC cartridges have 115 grain bullets.