New bcm uppers, look elsewhere - Magpul Industries Enhanced Ejection Port Cover

When it works, it works well. Problem is the odds of nothing being wrong off the bat are not in your favor. If you don't have the problems already mentioned here, one problem you may have to look forward to is your hand flying off the charging handle when attempting to charge the weapon - because the bolt carrier stopped cold at the port cover's inside 'pop-open groove' on three of four BCM upper receivers tested (the one it worked in was an older example; current-production BCM receivers have this issue). Never experienced anything like this after a decade of experience on the platform. Testing showed, using the same bolt carrier and charging handle in multiple uppers, that it was upper receiver-dependent. By contrast, the Strike Industries port cover had no issues in every upper tested.