5.7FN
I gotta comment on that.
The 5.7FN is not a deer round and likely only a moderate varmint cartridge. Interesting, yes. New and different, yes. Is the Sub 2000, light, novel and portable yes. Tremendous, no.
Ammo suitable for hunting with a 40 grain bullet (the best is likely the Speer Gold Dot) is listed at 1800 fps, (pistol?) and I'd give it 2300 fps from the 16" Kel Tec folding carbine. Might do more, but it doesn't matter. That is not "tremendous speed" but more like .22 Hornet speed. The Hornet was introduced pre-WWII. It has been used to take the tiny deer of Europe and the little antelope in Africal, but comes up short as a deer round in the 'States. The super light 28-30 gr projectiles in the FN are faster, and gain perhaps another 200 fps from the carbine, again an estimate. It still does not matter, it does not turn the 5.7 into a deer cartridge, and makes the FN 5.7 a varmint cartridge at best. The fact that it was developed as a PDW in a funky sub gun matters little in consideration as a sporting round.
It'd be a hoot on running 'jacks, softer to shoot than a .223/5.56 (but way more expensive) and really different.