I bought an M4 kit, the upper came assembled. I put it on my DPMS lower and the first test firing, it did a multishot runaway--freaked me out.
Tried my oly upper on the dpms, worked fine, then tried the blackthorne on my oly and had the same runaway effect.
Contacted them and shipped it in for replacement--the CS was good and even though it was beyond the "inspection" period (i did a function check and it was fine, but took a while to get to the range) they traded it and sent me a new one (but I was out return shipping).
The next one worked fine on the dpms but wouldn't fit on the oly...now I know olys are their own spec (putting it politely), or at least the one I have is. Anyhow, it worked fine after that but I ended up selling the whole rig. Wasn't thrilled with the product, but I'd give their CS dept a thumbs up. I might give them another chance on AR stuff, but for sure would on the other stuff (the AK kits I got were nice).