SIG showed me a couple of variants of their MCX range of carbines. Offered in 5.56, .300 and 7.62×39, the MCX is moving into production.
The Low Viz Assault Weapon above features a 6.75″ barrel in .300. Notice the folding stock which offers a very slim profile when folded.
This shot gives you a view of the right side of the weapon. The short suppressor shown offers above 5 db more than an MP5SD but in .300 at the same overall package size of the MP5SD. It also features a adjustable gas system but the subsonic ammo setting can run supersonic ammo but it will affect service life of the weapon.
Additionally, SIG offers an adapter plate for Standard AR lowers that integrates a Picatinny Rail that allows the user to swap out folding and collapsible stocks.