Well what is it? I want to cut down my 16" to a lenth that will give max velocity, full powder burn, and lastly short as possible. Any one have it figured out?

Well, if I recall, the Tommy Gun has a 10" (or 12" w/ the comp)  barrel and the Grease Gun has an 8".  I don't know if that maximized velocity/energy or was just the barrel length that was most managable.  Granted, this was on a SBR type weopon.

edit-
I chrono-ed my 1911 5" barrel vs a 6" barrel and the velocity went up by around 75-80 fps. I'm thinking a number between 8 and 10 inches should be fine.

Being used on an AR weopon, using a ACE buffer set up, an AR 45ACP pistol could end up just as long as the Tommy gun or just under.