whats M4 mean on a lower AR 15
I'm just starting to understand some of the terminology and what abbreviations stand for on ARs. A shop told me there stripped lower reciever are an M4. I asked what that meant, he explained, but I didn't comprehend. The guy was busy, I didn't want him to know I was green, and just left it at that. I used to think M4 stood for the design of the barrel. The kind w/ a bayonet lug and thicker walled at end of barrel. And I still kinda think that. Then I found that M4 stood for a flat top upper receiver.
Now I'm lost, being I'm was told that these stripped lower receivers are M4 lower receivers. So what makes a lower receiver an M4? Are stripped M4 lowers milspec? If what I'm desribing is true, can you put any type of milspec upper on this so called M4 lower? I'm hoping you guys can straighten me out on what M4 is about, and what parts it applys to. thanks.