SBRing a 300/221 - how short?

[RobRat]

I'm about ready to send in my SBR paperwork on an AR (starting with a 22LR build), and am wondering about shortening my 300/221 upper. I have a 16.5" 1:8 twist upper now with the gas block in the pistol position.

Some questions....how short can I go with a 1:8, and still get stable subs? (don't want a baffle strike) I'm assuming I could probably put more powder behind a 220 and still stay subsonic?

Any harder to get subs to cycle the action with a shorter barrel?