I have an AR in the A1 configuration, Olympic Arms; and a complete 9mm upper with collapsing stock. I've had these since '89 and finally got tired of two uppers but only 1 receiver, I wanted two complete guns. I came upon Joebobsoutfitters and saw a New Frontier LW-15 receiver on sale silly cheap. I got one and it fits and works fine one the AR and the 9mm. So now my problem, after avoiding it all this time I finally picked up some Tula 5.56 ammo. this stuff works fine in the AR in its original A1 configuration, but when I switch receivers, put the New Frontier (polymer) on the AR upper the Tula ammo will not function with the collapsing stock (I noticed that spring is shorter than the one in the standard stock), as it will jam at least every third shot with the fired case still in the ejection port and the next ctge. trying to feed and stuck at a 45 degree angle! I dug into my stash and loaded a mag with US LC mil. ammo and that stuff ran flawlessly with either receiver or stock. Tula ammo has likely been discussed here before but I just add my experience to the chorus of ummm-hmmms! By the way, I've never used anything but Tula in my AK and SKS and they never jam..