From my experience all lowers are not created equal in regards to mating tolerance to the uppers. Some are milled a few 1000ths +/- so one brand upper might work better matched with your lower. I Know that RRA sells "Matched" upper lower combos that fit tight.
For example, I have a SW MP15 carbine that I put the RRA NM trigger in (Best thing I could have done) and decided to buy a RRA Varmint upper.... They work together but the SW MP lower is a little loose with the RRA upper. My son's RRA lower is snug with no slop when I mount the RRA upper on his....
So when I got the ok from the "one who must be obeyed" to get a new, dedicated lower, I bought the RRA lower with the 2 stage NM trigger and it fits gooooood. Then there is the whole buffer tube to stock fit.....
On the triggers there are some very good expensive ones out there. (Some are not suited to Home defense) The RRA NM trigger is $70 retail and has a clean break and it is 100% reliable for HD use. Giessle makes some combat ready ones as well.
