A high percentage of AR's you see today are of mixed recevier combos. To echo again, as long as your receivers are built to spec then you can mix & match all day. Realize though, there is always a chance you can get one receiver that is out of spec & will give you headache trying to mate with the other. I never had a problem with this until I recently received an upper with an out of spec receiver that would not fit my lowers. It happens & this one was returned to factory. I went with another brand & now I'm back to no problems with fit again. Most manufacturers will put out lemons 1% (& that's prolly a high percentage) of the time but the other 99% of the time you have no worries. Some may be on the tight side & others can be loose depending on how much wear you have on one of the receivers. Mix, match & enjoy.