About a dozen years ago, Savage announced that the Model 25 Walking Varminter [THEE ugliest rifle I've ever seen IMNSHO] was going to be made in 5.7x28, but I don't know if they were ever released or not.
Just for $'s & G's, I ran the numbers on 5.7x28 vs. .221 Rem. Fireball & there was no comparison; the Fireball wins hands down. Not only that, but lots of available brass, including Lapua, more bullet selection [.223 for the 5.7 limits it to Hornet, Bee or Jet bullets].
But then, we're moving into the .222 realm.
Darn! Now you've got me thinking......
I have read in several sources, that the Euro makers of Hornets are almost all using .223" barrels, but that most of the ammo available in North America is loaded with the same .224 bullets everything else seems to use. Maybe that's wrong. Seems like there is enough variability in the barrel dimensions, it still seems to work out OK.
Not surprised to se that the .221 FB mops up the 5.7, pretty sure you'd be real hard pressed to match the FB with a loaded up Hornet either, but I think the Hornet vs the 5.7 might be a better match of near-peers, no?
On the other side, there are almost no end of the options once you want to start cutting down .222, or .223 brass and shortening those down. Seen a fair few sub-caliber (.12, .14, .17, .20 calibers) chamberings done that way
'Course, it wouldn't be that hard, I think, to punch a full thou out of the neck sizer on a 5.7 die set, or maybe just need to change out the expander plug for a .224 one, either... Though that gets to be more and more of a project... LOL! But I do sorta doubt you'd get much of a blink outta the reamer maker if you spec'd for the larger neck OD.
Be interesting to see if accuracy was at least adequate, firing .223 loaded factory ammo out of a .224 barrel!