There could be a lot of things causing this problem from ammo to incorrect magazine. I would go through your new upper and clean it well first.
Did you gve it a throu cleaning and lube job before taking it out?
I haven't shot mine yet, but will soon. It seems to be constructed well and all my other BCA products work fine. Matt is correct though and it should at least be lubed properly before firing. If I shoot it and the problem isn't solved I can review my experience.
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