I’m thinking about building a pistol in .300 blackout and I’d like for it to reliably cycle subsonic ammo without a suppressor. I’ve read that the PSA uppers will do that and I was leaning towards one of their uppers. Any thoughts?

I have a PSA 300 blackout pistolero with a 10.5 inch barrel and have had nary an issue! I've shot both sub and super.

Same. With or without a suppressor, my 10" PSA upper works with everything. Maybe it's due to the carbine length gas system.

If you go shorter than 10.5, it's probably going to have a pistol length gas setup.

With that, use an H2 buffer and see how it goes. I have had no issues with my 7.5 cycling subsonic without a suppressor with an H2 buffer. I haven't tried a standard carbine buffer.