My 7 PRC that is being spun up as I write this, is being built on Defiance LA Mag - not a RUM. We'll see how it feeds in a month or so. FWIW, my 300 PRC is built on a Rem 700 RUM action. Can't say one way or another whether the RUM action (ala a little bit different feed rail configuration) was/is actually needed - it's what I had available as a donor at the time. With the receiver cut for a Wyatt's extended box and follower, and the same installed, it feeds fine.
Probably so close it doesn't much matter either way on which will work.
I have a big RUM LA Defiance myself that I have sorta talked myself into doing in a 338 RUM.
I have had the action for about 9 months and changed my mind about 3-4 times since then
