On something like an ar-15 it is a good idea to go with a one piece cantilever mount as it allows you the necessary height, forward offset to provide you with proper eye relief as well as keeping the scope from being mounted to the handguard. Two piece rings are often a lighter option as well, which makes them popular for hunting rigs. For precision rifle applications, many one piece mounts are often available with differing inclinations which can be extremely useful.