100 rounds in. Break in period?

TL;DR: Whats the “break in period” on these uppers?

What’s up yall, put the first 100 rounds through my mat9-k, and I’m pretty stoked. The thing was an absolute tack driver and blew out a nice hole middle of the silhouette. BUT… I had a crap ton of misfeeds and failure to eject. It DID NOT like my sierra hollow points (I don’t like them either, tbh). The hornadys were fine. But the sierras would not feed. My bulk ammo from 2a warehouse did ok. I am using endomags, so that may be why I had issues. Waiting on my NFA lower to get a dedicated thing going so I’ll have to see then. Also broke the backplate, so Matador Arms sent a new one out the next day. TOP NOTCH CUST SVC! I want to get back to the range and try with a good backplate, 9mm lower, and no Sierra ammo and see how she functions. I really want to love this gun.