There are a few issues with the Endomags.
The ejector is too soft. Occasionally it would move in front of the bolt and jam it up. I'm thinking of either applying some RTV or epoxy to stiffen it up, making it almost permanent installation.
2nd issue is even downloaded to 28 rounds, it is super hard to reload with the bolt forward. If the ejector moves on you in the mag pouch, you will never get it to lock.
Since the ejector is on the mag, ULSC is a different process. You need to leave the mag in place, lock the bolt back, then remove the mag. If you don't do that, you have to manually remove the round from the bolt.