I'm building my first AR...first of all, thanks to EVERYONE for all their input. My question is, is there anything I while consider or any minor tweaking I may need to do if I'm going with a lower and an upper from different manufacturers? It's a BCM upper and a spikes lower. They're both 5.56. I didn't think I'd have to worry about it, but I read something in another thread about maybe running into minor tweaks I'd have to do? An I mislead or what? Thanks again..