We're getting off topic here, but this is the fun part. Yeah, I thought my 300BLK AR-15 was about the coolest thing ever . . . until I bought the bolt gun.

I did some testing with Titegroup for the bolt gun, and then Trail Boss, and finally settled on Trail Boss. Now I load 7.0gr of Trail Boss with either a Nosler 110gr Varmageddon or 150gr X-treme plated flat point. Both seem to run about the same, and yes they are SUPER quiet, like a freaking pellet gun! It's amazing. My AR is quiet I guess, but even suppressed I want to use my earpro because of the noise from the action, so the bolt action is a real treat.

And . . . yes . . . 7.0gr of Traill Boss is a compressed load. Yes, I know you're not supposed to compress Trail Boss because it can supposedly be unpredictable. However, I've never had an issue, so I run with it. This gives me 1030 fps (110gr bullet) and 1070 fps (150gr bullet).

It can go on forever really

The 300 Blackout Master Thread on arf.com is 61 pages long and 300blktalk has 242 pages of threads just on reloading

I don't know how much you've experimented, but for quieter suppressed loads, drop the speed down to 1,000 fps makes a difference from ~1,050, and down to 900 fps is even quieter than that. Below 900 fps doesn't usually have much advantage. Above 1,050 definitely gets louder, worse in cold weather. There is a chart on that floating around somewhere, but my copy is missing too many pixels to be useful.

Also some powders are significantly quieter than others, even within the same burn rate range, hence my suggestion for Green Dot which is quieter than Bullseye, Clays, Universal, Unique, WSF, or Trail Boss. American Select is another one that is quiet like Green Dot. The sound of the shot with those two powders has a duller flatter tone, while the others are a little sharper.
For low pressure subs that don't cycle a semi-auto, I've started to use Green Dot for everything from 9mm on up to 35 Whelen.