. Submachine Gun
. twenty five metres
. three dice
. four hundred starbux
. ten starbux
. fully automatic three
The Desperado example does emphasize that you build it around an existing bullet, and magazine, though the new Field Catalogue would make that a parabellum with three dice minus three.
There's not much difference the assault and long rifle receiver, at twenty percent more weight, except better recoil and heat soakage.
Traveller design mechanics really can't secure much advantage for the submachinegun, especially once actual costs get involved, and if you have access to higher technological level weapon platforms.
So, the question would be what you'd want the submachineguns for.
If you're partisan warfare behind the lines, automatic firepower at the squad level, and a mad scramble to produce firearms, assault rifles fulfill those aspects quite nicely, post Great Patriotic War.
I tend to view them more as deterrence.
If you can keep people from shooting you by just carrying or waving them about, they've served their purpose.
But four hundred starbux seems a bit steep, for what's basically a truck gun.
So, since recoil isn't an issue, we can move downwards from assault receiver with four recoil points, to handgun receiver, with two.
I bet someone is thinking about heat.
I think I've found a loophole.
Heavy barrel doubles the number of rounds in which sustained fire can be maintained before malfunctions become possible.