Building anything in RSAUM on any short actions is a huge mistake: the actual weight-savings are non-existing and you’re going to severely handicap the cartridges potential by forcing it to stay at basic .280Rem specs!Will a half inch in action length between short and long make that much difference? Eat one less cheese burger for lunch.
If you’re on a tight budget and want something that always works, feeds & ejects like butter and never lets you down (no matter what every other brainwashed keyboard shooter & his dog tells you), I would advice you to refrain from using any Remy platform actions! There are the usual 1/2 a dozen problems or more which you’ll have to take care off before you even get it up to other actions quality standards, which will severely increase your costs: instead buy a T3 Magnum action and use their standard .300WM/7RM magazines. I did lots of em; even made up an internal slightly ‘oversized’ mold-block, which I put inside the disassembled mag.. give it a few boiling water/cold water rinse treatments; this way I can expand the magazines inside wall space by .004” to .006”, so it fits & feeds the bigger diameter SAUM CASES perfectly! However: I no longer bother with the processe, since it will work fine too without doing this xtra bit of magazine expansion work! The only ‘mental’ handicap to overcome for some is that it will only take 2 RSAUM rnds in the mag., plus the one in the chamber, which brings it to 3 rnds: all you need as the one rnd only is needed to do the kill! Use a longer throated reamer which allows you to seat the bullets out to their full potential & the most ideal seating length, but one that still fits your magazine comfortably! You can further customize your built later on!
For those on a real small budget: there are old European gunmaking school-tricks to reuse your old 6.5Sw/7RM brl for such RSAUM/GAP/SST projects, with no adverse effects, other then loosing about 3/4”-1” of brl length, but sorry, I won’t get into that one on here!