I bought a 308 lower from Quentin Defense and began an 18 month journey to try to get a functioning gun together. Supposedly their stuff is "DPMS pattern", but no DPMS uppers work with their lowers. I went to DPMS and tried a handful of uppers on, then they pulled out a carton of stripped uppers and I tried on 10 or so of them with no luck. Couldn't even push the pins through.
I brought my Quentin Defense 308 lower receiver to gun shows over the next few months and tried different uppers on it. I tried 20 or so uppers with no luck. Some didn't fit, some didn't line the bolt up with the buffer tube, and some had a conflict with the bolt release.
I gave up this past Friday, went back to Quentin Defense and tried to get an upper built. They talked me out of it and into a prebuilt upper they had on the shelf. I noticed it was well used, but sold to me as new. I figured it was no big deal, I'd finally have the thing working. I went to put my silencer on and couldn't. I tried another one, no luck. I took a look at the muzzle threads and shoulder area. Don't drink and lathe. There is no shoulder on the muzzle for a muzzle device. There are a series of bulging tapers that prevent muzzle devices from being used. Also the crown is a weird, deep 45 degree cut.
I returned the upper and sold them back the complete lower for $50 because I couldn't in good conscience sell it to anyone. The lower won't work with anything but Quentin Defense uppers, and those either don't work or don't exist. I wasn't going to sell that lower and force somebody to buy a Quentin Defense upper. There are like 30 alternatives and any one of them will work.