I ordered a Ruger American Ranch Rifle in 7.63x39 for a primary host. I shot it on the 300 Black Out AR and it's a little loud through the semi-auto action. I have a ton of crated 7.62x39 from the 90s plus some Yugo stuff that is "corrosive". Next thing with be a straight walled caliber. I can't bring myself to thread my 1895 SBL.
I found the gen 2 in 300 blk the other day, place wanted 630 for it. My very local lgs said he could get it for 525 when it came back in stock with his supplier. Holding out for that, hoping that will be true. It was a good feeling little rifle....
400 legend sounds like the perfect straight wall round. 350 L is pretty good, 450 BM is a little too much bullet, and too much drop. The 400 L looks to be the sweet spot. They drop less, but have knock down power without extra kick. It will fit in an ar15, like the 350 and 450, using a 6.8 SPC bcg. So it needs a barrel which there's a couple available already and then the ar mags. I'm sure for 3 RDS that you don't need a specific 400L mag.