I was thinking in terms of a handgun that can share ammo with the rifle.

Lighten the crye gun to 1kg and usr shared mags for sidearm and long arm

That's a taller order than we can actually pull off lol!

Don't get me wrong, that would be awesome.

It's just sorta beyond our capabilities to produce.

Also, sharing magazines seems like a really cool idea on paper but it's not necessarily something worth making the compromises you'd have to make to get there.

If you go down that road you either end up with something like the gwinn arm gun/Colt imp (picture below) 

Or the Magpul PDR 

Neither of which are going to weigh 1 kg or holster particularly well.

Also 1kg empty weight even without a magazine doesn't give you much room to work. My two favorite machine pistol designs of all time (Because I think they got closest to hitting the ideal) are the Colt SCAMP and the Robinson model 11 neither of which met your weight bar. (guns.fandom has good Robinson model 11 information)

Yeh i was more curious about the feasibility from a mechanical perspective....a hypothetical automag action with crye angled ammo

It just seems you could lose strength by going from the normal 'mac 10' shape to a small movig slide

You could use the saved length to add more.barrel and avoid thr flash problems of 556

Oh you almost certainly could go to a more automag like action and that could be pretty interesting.