Our No. 1 customer complaint has always been “I only get +2!” That’s because we intentionally build the cavity in our magazine extensions slightly undersize to compensate for any possible misalignment between the mag tube and the extension. The downside to that is that occasionally the follower will hang up on its way into the magazine. This video shows Robin Taylor assembling an M&P extension that had this problem, and fixing it in real time.
https://www.youtube.com/watch?v=cKh_Mqj1rb0