>>64248813
I mean more because it's something that people already experimented with back in the day. .22 Spitfire / .22 MMJ / 5.7mm Johnson was .30 Carbine necked down to a 40gr .224 caliber projectile, as a lower cost way of making an SCHV rifle, you could just convert existing M1 and M2 Carbines with a new barrel and I think an added bullet guide.
Actually worked out pretty well, too, but it simply wasn't as good as the AR15, so it never went all that far, and on the civilian market end, people greatly preferred the cheap .30 Carbine surplus ammo.

An M1 or M2 converted for the 5.7x28mm pistol cartridge though is interesting in itself, it might even work in the same magazines (though they might also require a spacer block or something). I like the idea of one like as a handy varmint rifle.