If you have a M4 height front sight base, you should not have a problem. If you have an A2 height front sight base, you will need a slightly higer front sight post. Bushmaster requires the higher post. I have a 16" Bushmaster pre-ban A3 upper with a #38-EX Swan Sleeve on it, which includes the #40 BUIS. Outstanding unit. I use it for mounting my Aimpoint ML2 with a A.R.M.S. Comp II M68 quick release ringmount. I cannot say enough good things about A.R.M.S.