I had a 458 WM barrel set up on a long action with a 300 WM magazine and it fed 458 wm fine but its an old magazine.
It also feeds mostly anything alse, IE 30-06 etc.
I also have a 30-06 magazine and maybe you have the wrong follower.
The 30-06 follower has a thin coil spring on it.
It probably would not have enough spring pressure, to push the
magnum bottom shells up. Never have tried it with magnum shells.
The magnum follower, is a leaf spring shaped like a W.
Midway will take anything back if you have the wrong follower.
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