A Tale of Two 350 Rem Mags

NOTE: The rounds used to check feeding are dummy rounds I made with no powder or primer.....don't try this sort of thing with live ammunition! A Remington 600 in 350 Rem Mag traded for a 600 in 6mm has joined our household. Like a lot of earlier 600's it feeds a lot less smoothly than the Remington 660 in 350 Rem mag I have had for the last 27 years. Why? Is there anything that can be done about it?