What cartridge?
And have you considered a shorter barrel?
I'm looking at a similar setup whe I get my stamp(next month?).
I'm thinking a large caliber like a .44 or .45 at subsonic.
.44 mag it is Holly Wood Quiet with special in it.I will develop some loads for it.
I may get a 10” barrel to see how that goes.I will not cut this one down it shoots to good.
4 rounds at 60 yards.