I feel like maybe we're doing different things here. I'm looking at the gun as described in the graphic, while you're looking at what a space suit gun ought to be.
The gun as actually seen in SAAB was never well defined, but I think it's clear it fired a chemical projectile, and the assumption was that it was something around 5.56mm (possibly caseless, because the prop guns weren't seen ejecting cases very often). Caseless actually seems like the worst possible option for a space gun, because it has no brass to carry the heat of firing away from the chamber, and no plastic to insulate the chamber from the gun gas.
I think a hypothetical space rifle would look a lot different. It probably would need a different sort of bracing that a shoulder stock (and definitely not that weird wrap over the arm thing), because it's hard to shoulder a rifle in a realistic space suit, but you definitely want a three-point hold (shoulder, trigger hand, off hand) to ensure stability and repeatability of aim. You want a sight that works without parallax, so at least a red dot or better a camera linked to a HUD in the suit. And you want a fairly large magazine because changing it is going to be harder in a suit.