I have a savage 110 I converted to .458 win mag, it's just a fun project, mostly. I'll shoot deer with it this year.

Anyway. I replaced the mag and follower with ones made for the 458 win mag. If the shells are loaded in, staggered back and forth, it works fine. But if the last shell is not forced over to the side, but is just left in the middle after it is snapped into the mag, then it rides up between the feed lips as the bolt is pushing it forward, and it jams between them.
I know one solution is just to make sure the shells are in there properly. But it seems to me the thing shouldn't jam up under most normal conditions.
Am I wrong about that?

I'm not exactly sure I understand what your talking about, but if its jamming between the lips maybe they need to be opened up a little bit? I just did some work on my 375 savage and now its feeding real slick. After tuning the magazine and polishing the feed ramp and the magazine lips it doesn't have a problem feeding fn solids.

What caliber was it to start with? Joe (welcome) is on to something. If you can snap it in to the mag, it should also come out, right? If you are forcing it to go between the feed rails, then open them up just enough to let the brass come back out. If it stays in the mag and is still in the middle, seems like your feed rails are too narrow. Rounds should not stay in limbo; either right or left.

You didn't get a centerfeed follower instead of a staggered feed follower did you? The centerfeed followers are dead flat on top. Staggered feed followers have the typical high side on the left and low side on the right looking from the butt end of the rifle.

Just a thought. I've gotten mixed up parts from vendors before.

Jeremy