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I've also had issues where the outer barrel sits too far back in the upper receiver. If you look inside the upper from the bottom, the outer barrel should be flush with the front "wall" of the upper. If it's sticking into the receiver 1-2mm too far it won't line up with the mag properly & you'll have issues with the lower & upper mating properly as well as the mechbox sits against the hop-up wich sits against the face of the outer barrel.
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