I have an OLY SGW lower that I put a JT M4 upper on and they did not fit together correctly. The takedown pin holes lined up, and the pins went through, but the upper protruded into the front of the mag well by about 1/8th of an inch and will not allow a mag to be seated.
I have had this upper on both an Essential arms, and another Oly lower without any problems, but have never used this lower with another upper, and do not have one availible to try. Has anyone else had this problem?
This did teach me a valuable lesson though, make sure everything works after fully assembling the weapon. I made sure that a mag could be seated after I had put the lower parts in, and did a standard mechanical function check with the upper on, but never tried to seat a mag in the fully assembled rifle until I took it out to fire it. What is even worse is that I was going to sell the rifle, but when it didn't sell by the weekend I decided to shoot it. Boy am I glad that it didn't sell, last thing I want is to give someone a bum deal!