Did you mean the second round would actually eject with the fired hull, or just pop up and lay in the action?
My limited experience with military Mauser actions feeding std belted magnums is that it's a combination of things, starting with the mag box maxxing out at approx 3.3" C.O.A.L. A .338 Win can be short loaded to fit, but the round is still fatter than the the Mauser round. If you don't grind the rib down on the follower, it should hold the magnum case far enough over to the side of the well to keep it from popping up, and the rails should be holding the body of the round down long enough to let the bullet ramp up into the back of the chamber. Relieving the rails gets tricky, but may have to be done to allow the case to ride up and angle over to let the bullet nose start into the chamber and the base start riding up under the extractor at the same time. If you can push a loaded round straight down into the well, you have enough clearance for the round to feed, but you've got to prevent it from trying to single-stack. Three rounds is all you should load. The first round should ride naturally over under the right side rail, and then it should hold the second round over under the left side rail, then the top round over to the right again. The second round may be prone to popping up and harder to get to reliably feed, the first and third rounds should be more consistent. Sounds like that may be what's happening to your gun since you can chamber the first round O.K. I assume that the bolt face is opened up to about .535-.540", and the lip of the extractor has been worked back just enough to allow the round to ride up the bolt face and under the extractor with just enough snap to hold it on the bolt. It should snap lightly and cleanly under the extractor or it may just cock up and angle over and jam. What makes the Mauser scary to me is removing metal from the rails. If you mess up, you can't put it back. Working the follower and the extractor are low stress because you can replace them if you mess up. If you have a commercial large ring action or a military 98 action that feeds belted magnums, it's good to use them as a template for modifying the follower, rails, and extractor on a conversion, or to figure out why one is not feeding right.
I'm not recommending it, but you might try grinding the front end of the bullet follower rib down lower than the back to allow the front of the round to shift left and right easier. Also, chamfering the top inside edge of the mag box might let the fatter case get a little further under the side rails. I'm guessing on this, because I haven't tried either one.
I don't know if any of this helps, but maybe something will. My personal opinion is that a Mauser is a tough first project.
Good luck, Tom
