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Having a bit of trouble with a "Colt 604" build. I've assembled a few AR's and haven't run into an issue like this one. The BCG won't clear the end of the top receiver and sit flush. At first i thought something was in the chamber but its clean I also dropped a different BCG into the gun and it sticks out the same length. I am unable to hinge the upper shut. The lugs still rotate, and it head spaces. My thought is that it's an issue with the barrel and chamber. For those that ask the barrel is a Brownell's 20in and BCG is a Browells chrome lined retro one. Not sure what else to try other than reach out to Brownell's and try to get a replacement barrel (the BCG fits in a couple other ARs)