Do you have a pet load that is a clean burner for shooting subsonic in a suppressed AR

I only use 5744 and 1680 in 300 whisper/blk - but of powders I use that seem cleaner than others are the VV320 and VV340 I have tried. So you might try N130 or N133 powders by same company.

I use Vihtavuori N120 for my Subsonic loads:
10.7 grain; C.O.L.: 2.255"; Velocity:985 fps; Barrel Length: 16", Pistol position gas port (dia. 2.0 mm).

Shoots really clean and flawlessly

Have you tried lilgun?

Generally speaking, any low pressure round is going to be dirtier burning than a high pressure one with all other factors being the same. The trick is to find a powder with good ignition characteristics across the range of pressures you are likely to experience. For this reason I would highly recommend at least experimenting with lilgun. Another good suggestion might be IMR4227, as most IMR powders burn hotter than ball powders and will tend to burn out fouling. I don't have load data for the 300 AAC for either round, but I have loaded these powders in a wide variety of other cartridges and would recommend them for this use.