I have a MP15 sport complete upper receiver I have on a LMT lower that I just recently finished building. 150 rounds in 30 minutes with no problems. The MP15 sport is there cheaper ar platform that used a melonite barrel with no forward assist or dust cover. From the research I've done the sport has been put through some grueling tests and performed very well. For example one guy ran 12000 rounds with out cleaning it, just some minor lubrication and he did not have one malfunction until he kicked a tone of gravel in the open chamber. Misfired twice because the firing pin was gummed up. But after he put some oil in the bolt carrier group it started working just fine again.