The 5.7x28 FN
This diminutive Belgian import delivers smallbore steam from a very small package.
The 5.7x28 was developed by FN in the 1990s, and there is some pretty good evidence that it was originally developed for some real or proposed military application. At one time (perhaps still) there were four loadings with bullet weights ranging from 28 grains to 55 grains that included a tracer and a subsonic loading.
These loadings aren't available for civilian retail sale. They do suggest that FN has certainly considered other uses, other than just a simple pistol cartridge. And indeed, it was originally developed for the P90 Personal Defense Weapon.
Don't let the "5.7" part of the name fool you. This isn't a .23 caliber. This cartridge uses standard .224-inch-diameter bullets, the same as almost all .22 centerfires. As the picture shows, the 5.7x28 is what could be called a rimless Hornet--actually, it looks more like a rimless K-Hornet. If I could accurately predict which new cartridges would sell and which wouldn't, I'd be rich. Still, I think the 5.7x28 has good potential as a modern light .22. I would also suspect that by the time you read this somebody will have necked this neat little number to both .17 and .20 calibers. (I've done enough of that sort of thing and will leave this one for someone else).
The only factory handgun being offered in this caliber is FN's Semi-Auto Pistol. It's interesting that while the factory is only producing 27- and 40-grain loads for civilian sale, the barrel twist in the pistol is a very tight 1:9 to stabilize bullets up to about 70 grains and further suggests that the factory is still thinking about other applications down the road.
Bo Clerke made us two barrels, one five inches long and one 22 inches. At this writing, no one is building production rifle barrels in this caliber, but that will probably change overnight. It would sure make a great single-shot pistol. I know several companies are just waiting for a little demand to develop before cranking up their own production.
Today's ammo situation is interesting. FN is building some ammo with 27-grain aluminum-core bullets in Belgium, and Fiocchi is making some with 40-grain Hornady V-Maxs in Missouri but only to be sold through FN. So that's the only source of cases for the present. Both the FN- and Fiocchi-built ammo styles employ staked-in primers in the military tradition. They both use Boxer primers and are easy to deprime but require reaming the residue of the staking off the mouth of the primer pocket before they can be reloaded. That's a bit of a drag, but you only have to do it once.