Did you do a fitting?? Many drums require a bit of filing to get it to fit properly. Check youtube search MD arms, he has a video on how to fit it. Just be VVVEERRY careful not to take off to much material.
Yes, it fits just fine when not loaded. I already filed in the areas needed. Could it be S12? Maybe not enough oil inside? IDK. I can pop the drum on not loaded. But loaded, even just a few rounds, doesn't want to go.