How much shorter does the barrel have to be to be able to use the bayonet. I am thinking about having one cut down with the FH permenently attached so the bayonet will mount and it will be 16+"

Think that can work ?

I believe the barrel has to be [i]longer[/i] for a bayonet to fit properly on a mid-length.  16-11/16" or something like that.  On a regular 16" with the mid-length the ring on the bayonet sits in the middle of the FH, instead of at the back like it's supposed to.

If you have the standard CAR-length HGs then 14.5" is the proper length for bayonet mounting.  That's why the M4s are that length.