"Accurate Mag" in the US make a 10 round, steel, .223 / .204 Ruger etc mag.

The trick is that, like the old Sako 75, the .223 part is an insert in a standard 7.62 / .308 "shell". this simplifies things for folks building on Remington 700, Surgeon, Savage 110 actions, etc.

Because the AM mag is set up like an AI mag, (double column tapering up to a single feed), it may be the way to fly.

Alternately, rustle up some after-market .223 mags for the old Remington 788. These are fairly simple, single column affairs and could be easily black-smithed into a larger "shell", be it a No4 or whatever. Ditto the old Tikka mags (and clones) of similar design. Regarding the "shell", the somewhat "squarer" Indian 2A1 magazine case might well be better suited for housing such inserts, and shouldn't take TOO much fiddling to lock into a No4.

Yet ANOTHER .223 mag alternative, for those living in reasonably enlightened places, is the single column 5.56 mag intended for the "politically correct" AK-based sporters.