ar upper and lower question

[vulcandeathbuny]

I am new to the ar platform. My question is will a milspec upper fit with a comercial lower and vise versa?

 

[HOOfan_1]

Milspec versus commerical is usually used to compare the diameter of receiver extension tubes.

At one time Colt had pivot pin holes which were of a different diameter. Colt's commercial rifles had a larger diameter hole. There are adapters out there which will allow you to use the old Colt's with Milspec parts though