I have three different 300 blackout's. The first one is a Ruger American rifle bolt action. This is definitely the way to go if you want to shoot subsonic suppressed. It is much quieter than the AR's.
The second rifle is a BCM 16 inch with standard forged receivers. I get the desire to spend money and make a really nice rifle, but we're talking about something that has a useful range of 300 yards really. The ultimate in percision is not that beneficial for this caliber in my opinion.The 16 inch barrel is for a longer that is necessary to take a vantage of all of the powder capacity. And a pistol length barrel makes a lot of sense, other than for hunting and legal issues.
A pistol in this caliber is a different animal altogether. To me it is a very capable gun for relatively short range hunting and self-defense. With a 9 inch barrel, and a folding stock, mine easily fits in a backpack.
I built mine on a mega arms upper and lower. I'm rather glad I went this route for purely aesthetic reasons.
Go to rainier arms and find whatever you want, but I would consider weight before precision given the application.