Have you shot this rifle yet?
I think you're trying to hand-cycle it, it's never been fired, and the buffer spring wasn't locked onto the buffer. Sometimes, you need to work for a minute to seat the spring on the buffer. I'm betting it's not seated on the buffer, thus causing extra length, and you can't lock it back by hand. The first round down the barrel will seat it, i guarantee that.
That paint is badass, by the way.