AR barrel length???

16 is ok... even a 14 with a 2 inch muzzle break attached is a go. wait what is a pre ban upper ... do you mean an upper without a bayo lug or flash hider... if thats what pre ban upper means then yes.... you are good.
 
16" is fine. If it says pre-ban upper, it most likely has a bayonet lug and a flash hider, which are not MA-happy. You would have to change those before putting it on your lower unless your lower is pre-ban.
 
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