LRPV Repeater, Caliber Change???

I need some info if someone knows. My nephew has a LRPV repeater 6mm BR. He bought it as a stocked action from NSS with a McGowen 8 twist barrel. I installed the barrel for him & also purchased locally a 12 twist 6mm BR RBLP LRPV for myself. I'm going to trade him out of his gun and put the barrel on my gun for heavier bullets. I have a 260 LRP barrel in the safe. My question is, if I put the 260 barrel on his gun how much trouble will I have getting the mag to function properly? Also I can't remember is the repeater stock a 3 screw stock? It been a couple years since I put his barrel on & I don't remember how the stock set up is with the removable mag. I also was wondering if the mag will allow long C.O.L.s. I may be better of buying another single shot. Anybody done this? Thanks

Iv'e never owned either rifle so take this with a grain of salt. Being as it's a repeater the middle screw would be where the mag would go?

I wonder if a 12 Long Range Precision .260 mag would work. It looks very similar to my .223 LRPV DBM.

I guess I'll check the throat on my barrel with the Hornady gauge and load up a dummy to that length. I can then check and see if the 6mm BR clip will accept the round. I was just hoping someone here had tried or allready knew. Thanks.