5.7 AR upper problems, need advice.

[Superhouse 15]

OK, I'm working overtime for my real job and can't get to the gunshop today. Last time I was there, we put together a lower for a customer who has a 5.7 upper. We fired it a few times after assembly and had no issues other than the heavy buffer it came with is noisy.

The customer is there today and nothing but problems. Double feeds, the occasional slam fire, missfeeds crushing the cases, and now, light primer strikes. I've been offering advice from afar and no success yet, so I'm asking the same from you all. Anybody have any experience with this platform? So far they have changed out the carbine spring for a rifle-length, and switched the heavy buffer with a lighter one, success is limited. Ammo is the blue-tip FN, stock 50 round mag blocked to 35 () rounds. Well lubed, brand new Spike's lower with a DPMS parts kit assembled by skilled gun monkeys.

[DefensiveCarry.com]