Firearm frame or receiver. That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel.
AR的下半身机匣,首先就不满足后面这一部分"and which usually threaded at its forward portion to receive the barrel",因为它不能插枪管。但是你说"usually"是不是有例外呢?那就要看"and"前面那一半。我对英文的解读是这样的。"provides housing for"下面这几条是"and“的关系,就是以下三条必须都满足: