Depends on who's military and which version?
US Military M16A2 or M16A3 you order an A2 upper and a 20" barrel.
US Military M16A4 or Canadian & Danish C7A1 order a flattop with a 20" barrel
Canadian Military C7 order a Bushmaster A1 upper and a 20" barrel
US Military M4/M4A1 or Canadian C8 you order a A2 (early models) or flattop upper and a 14.5" M4 barrel (you will a permanently attached muzzle device like a brake to make it legal).
(Bushmaster already makes a post ban M4 clone complete with faux telestock).
Edited to add:
To make the 20" rifle WEIGH properly you will need to send the upper to Kurt Wala (see the bottom of the industry section) and have him trim under the handguards to an A2 profile (saves 1 lb of weight), or buy from Gunsmoke (www.ar15.tv) the Bushmaster Govt Profile barrel and have them remove the bayo lug and permanently attach their 4 slot muzzle brake (looks alot like an A2 flash suppressor).
The Bushmaster M4 barrels are already the proper profile & weight (do not get the 14.5" HBAR barrel).