The Contender is not big/strong enough for the hot .50 cals. It's limit is about the 50-70. However, the Encore will handle the 50 Alaskan, 500 Maximum etc. JD Jones of SSK is your best bet for talking through this one. I would stick to the rimmed cases. The 50 Beowulf was designed for the AR15 and has a rebated case. The Ruger you describe should be OK but my personal preference would be to have it built on a High Wall pattern. I would build the Ruger #1 into a 500 NE or similar.