Making carbine rifles could be done with using the appropriate weapon name (points to caliber/ammunition, also keeps stuff canon-friendly). Carbines are effectively rifle -caliber weapons in shorter "chassis", so you get the power of a rifle in a SMG package. Double the cost in credits.
Carbines require two hands to operate due to their rifle caliber (higher recoil than automatic pistols/small caliber smgs) and the size of the stock, receiver, and barrel length.
Suggesting pistol ranges for these weapons, medium capacity (high capa clips could be x2 the cost and harder to obtain), and normal autofire rules (halving the range).
ex. Autotarget carbine, item lvl 2, cost 1500, 1d6 P, cap 10, range 30ft, analog, automatic.
That would be your "street-level" weapon that every thug can get off the black market. Like a homegrown AR-15 variant, that has been tooled with to make it fire at full-auto. Smaller caliber (9mm or so) fit into pistol capacity clips. You can double or triple the capacity on this thing just by adding length to the magazines. Should be relatively difficult (DC 15) gunsmithing roll/ blackmarket purchase. Not a game-balance killer, just something cool to add for starting level characters. Most of the time these street-level modified carbines will not feature attachment rails, because those things are milspec stuff, and thus not available to the general public.
The next level would be military stuff, where everything is optimized, from range down to the handling of the weapon.
ex. Combat carbine, item lvl 15, cost 33,000, 3d8 P, cap 24 (extended), range 60ft, analog, automatic, railed
Think of your modern military rifle-caliber carbines and this is the M4 of the Starfinder universe. I plan on giving this to all my police/military personnel in my games. Made of light metal and some version of polymer materials, they are light and sturdy, with attachment rails and usually a foldable stock.
Benefits of this weapon platform are increased capacity, added autofire capability, and concealment (regard the weapon as a small arm). If a carbine rifle is fitted with a collapsing stock, increase the difficulty for spotting the concealed weapon depending on the situation. Your standard career criminals will definitely want to get their hands on these.
Finally, halfway between a carbine assault weapon and a sniper rifle is the DMR (or Designated Marksman Rifle):
Take the Combat carbine, convert it into a heavier caliber, increase barrel length and weight, add a medium-range scope, remove autofire capability, add a bipod if you want and you have yourself a military DMR.
ex. Combat DMR, item lvl 15, cost 49,500, 3d10 P, cap 10, range 90ft, analog, railed, sniper (250ft).
This is the SR-25 of the Starfinder universe. Somewhere between a battle rifle and a full sniper rifle. This fits the niche for operatives, and would-be assassins, especially if mounted with a collapsing stock and removable scope attachment (for close quarters combat). Increased capacity for longevity in the fight.
