Are you trying to make a MK18 by any chance?
275mm is the total length of the product, 10.3" is the nominal length of the outer barrel part.
You don't want a 7.62 rail for an M4. Get the DD 9.5" (especially if you're going for the MK18 look).
As for the QD KAC silencer, you will need a bit of space between the end of the rail and the rear end of the flash hider as the suppressor fits over that.