I have seen one Core-15 and one DB15 (Diamondback) being shot in person. The CORE15 trigger was disgusting but thats a simple fix. The DB15 was having problem after problem with several types of rounds... FTE' FTF, failure to lock the bolt back, casings getting stuck in the throat (would that still be classified as a FTE?)
We tried USGI, PMAGS, lancer mags, Federal, Americna Eagle, Winchester, Tula and Wolf and have the same issue with EVERY variation.
He bought the DB15 during the height of the Sandy Hook ban scare and paid way too much for it. It looks cool, but is not reliable. Given the issues I had with my DB9 and the terrible CS I got, I reccomend he cut his looses and get another brand. There is no better time then now.
To expand on my previous reccomendations. I have and have had several CMMG products and love them, RRA and PSA always! I may get tongue lashed for this but I had an Olympic arms I bought in 1996 that functioned flawlessly. The only gripe I had was the barrel was not threaded.