You do know, that the FN5.7x28 as well as Ruger 5.7X28 can be purchased in the US, right?
So either the AAFES (Army Air Force Exchange Service) is selling illegal arms, and I am going to jail on a weapons charge, or your dining pleasure will be a flambe of crow
Or are you going to continue to be a pretentious ass?
And that is the point.
5.7 was designed to make up for lack of rifle-scale power via a high-rate high-capacity full-auto in a reimagined carbine package, putting a lot of holes in a small space rather than one big hole. It’s the best you can do in a carbine with a pistol caliber - and requires full-auto to make it up by volume.
If you’re not aware: full auto is, for all practical purposes, illegal in the USA. Like seriously, 10-years-in-prison and $250,000 fine, illegal.
Without a full-auto platform, 5.7 is merely a respectable high-density pistol round.
If you’re looking at a carbine-sized platform, and are limited to semi-auto, there’s no reason to not use a rifle caliber. Yes, SBR is legal (your jurisdiction may vary). If there is, I’d like to hear it - without ad-hominem insults.
Still not seeing why you’re rabidly arguing off-topic.