Generally for entry one of the tests that the customs officer will go through in the check list is that it's fully clear PROHIBITED parts (not everything just the prohib ones as stated by the law). That generally means the "body" or rather the part that carries the serial number on it. That is; lower for AR pattern rifles, upper for M1A, upper for G36, and upper for MP5.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE