I am trying to decide if I shoud send my DPMS post ban upper to ADCO and have it turned down to a gov't profile M4 with FH to a total of 16" and fixed in place.   Or just buy an M4 DPMS pre ban barrel and new B/BC.  I realized that after I install the A2 FH the barrel will be  slightly longer than 16".  If I could I would just go ahead and buy barrel that is already completed in an M4 stlye the exact way I want it and the put the old post ban back for a future build.  Anyone know who has the barrels like I want for a reasonable price.  Also what's the skinny these days on B/BC's.  If I remember back in armorers school that it was a no/go to mix a bolt with another rifle.  I figured that was because headspace was not done at a unit level to verify the weapons safety factor.  Can someone elaborate.

Simple answer.

Keep your postban as is.

Buy the new preban upper. 2 uppers, 1 lower. In the next 2-3 months buy another lower and voila, 2 rifles.