Standard rifle type powder can take more than 19" of barrel to achieve a full burn, and in a normal unsuppressed rifle with a short barrel, most of the gun powder not burned when the bullet is still in the barrel, is burnt after the muzzle, hence a large fire ball instead since the unburnt power stays in the stream of fire to be burnt.


Now add a suppressor into the mix, and that unburnt power is blown/dumped into the first expansion chamber of the can, where it can build up over shots from not being burnt, and now since the can is adding back pressure back down the bore after the bullet has left the can, the unburnt debris in the expansion chamber (and even some of the baffles as well) is now blown backwards down the the barrel towards the chamber with the back pressure of the can.



So the problem is not so much the rifle, or suppressor since is does sound like the rifle is unlocking and stroking correctly, but the gun powder being used for the load instead.  Bluntly, if you are going to be shooting a short barrel rig with a suppressor, then really need to start loading for such, instead of using loads designed for a longer barrel that allows the slower burning powder the needed amount of barrel length to burn instead.

Also, if you are going to use H110/296 (its the same gun powder) to base loads on, then you need to play close attention to the the volume fills of the power in the cases.  With this power, never use drops that will not take up at least 90% of the case volume of the load.  If you are dropping less then 90% of case volumes after bullet set, then you need to add in a packing above the powder/below the base of the bullet to keep it compressed tight  to the primer side of the case (TP will work fine to keep the power tightly packed and take up unused air volume of the case).

To add, If you have every run a MP-5SD, which is designed to run on +P ammo and to keep it subsonic since the barrel is ported right after the chamber to dump working pressure of the barrel right into the expansion section of the can as the bullet is still moving up the barrel, they you are aware of the mess/amount of fouling that is not only dumped into the reflex can, but back into the action via the back pressure of the can as well.  At least with it, it's using pistol powder which burns faster and more easily to ignite, but still a messy rig neither the less.



So on that note, the rig is going to run on the dirty side with a supressor from the can pressure wave blow back to the chamber, and now it just a mater of finding a power that will burn more fully, so less unburnt power is being blow back instead.

ACC took the 300 whisper, and got it SAMMI standardized to call it the 300 blackout instead.

As for as sonic loads, since it now has a SAMMI standard that the ammo manufacturers can use to reload it, and say for a 14.5" M- barrel, all would good in the world if that is the barrel length you are running.

Now comes subsonic loads, and since they are not loaded to even close to Sammi's 55K working pressure (but may be designed with enough port pressure to cycle a 14.5" barrel action), this is where things get tricky with a short barrel instead.