I'm thinking about doing a build using a hogue FF tube as a supressor tube. Plan is to chop the 20" 1x9 barrel to 4-4.5 inches, thread it for a brake as the blast baffle, then use the remaining ~8" after the muzzle for K baffles. The gas tube holes in the barrel nut will be plugged. Anyways my question is- if I'm using a drop in conversion with chamber adapter, and the barrel is short enough to keep everything subsonic, will there be enough length of rifling left after the adapter to stabilize from 36-40 grain bulk or 60ish subsonic? Anyone using chamber adapters in really short 1x9 barrels?

I just put together a 7.5" 22lr conversion, seems pretty accurate so far.  And I've tested CCI standard velocity and it stays subsonic

CCI SV is sub sonic from rifle barrels. A 4-4.5 inch barrel will keep all bulk pack ammo subsonic. I have TACCOM 4" barrel in the ULW and all steel. I like the steel a little better but both work great. I runs Huntertown Kestrel on mine and it make for a quiet compact gun. A lot of fun to shoot.

MAHA

4-4.5" should be enough to stabilize in 1:16.  There are plenty of people here that use those lengths with cans (I have a 4.5 CMMG that i use with my sparrow and it works great).  Pistols like the sr22 go as short as 3.5" with no issues (that I'm aware of).

1x9 is faster than 1x16 so it should help with heavier sub sonics. And 1x9 because that's the twist rate of the 5.56 barrel I would cut down and rethread to save money, because I'm trying to do the project all with components I already own so I only have to beg the wife for tax stamp money.

556 barrel is slightly oversized for 22lr, which will effect accuracy.