Why would they make a 1-8" twist for longer/heavier bullets and then put a block in it so the cartridge OAL is limited to 2.292"?
I looked at the short action mags for .22-250, .243 etc. at my gun shop and the short action measured close to 2.750", all I could find to measure it was a ruler so that number may vary a little with calipers.
A member on a different forum picked up a .223 last night and measured the mag box with calipers and got 2.292". So evidently Ruger is making a different mag. for the .223.
No 75gr A-Max's unless you can modify the box.