I already had things like these in my mind for some time, and now I finally managed to get my ass up and started to actually draw them. xD
This one will be the start of a series about realistic weapon concepts focusing on "potentially disposable weapons": reloadable infantry weapon systems that are designed to be cheap and with a short lifetime, but reusable if the firing rate is secondary (because other soldiers can take over and fire their weapons while the rest reload).
The DM-301 is a lightweight and compact rifle grenade launcher system designed for short-ranged urban combat. At an empty mass of 990 grams it is just as heavy as some WW2 hand grenades, yet it can fire a small rifle grenade (weight 270 grams, with a 150 gram filling in the HE version) up to 267 meters. It's muzzle velocity is 53 m/s, which allows an effective range of approximately 100 meters, more than enough for urban combat.
Except for the barrel and the breech, the DM-301 can be made of plastic and thin steel sheets, which may be pressed and molded in large numbers. Both the barrel and the breech are made of steel, but the barrel itself is a smooth-bore version, as the grenade is launched with a blank 5.56 mm shell. Since the steel is just of a medium quality, the barrel would be only good for a few hundred shots before it would need to be replaced, but this is more than acceptable since rifle grenades are not used often enough to actually stress out a launcher very fast. The barrel is screwed into the dispenser tube (which keeps the spring steel fins folded together in order to lower the weapon's overall diameter during transport) with the grip made of plastic, and is locked onto the breech with a simple bayonette mechanism. For firing, the soldier grabs the launcher on both the front shaft and the pistol grip, twists the barrel to the right, and pulls the aft of the weapon backwards, ejecting the shell inside the barrel. Then he puts a new blank into the barrel, locks it, places a new rifle grenade into the dispenser cup, and the weapon is ready to fire again.
The firing mechanism is a simple double-action mechanism similar to the one of a revolver, but simplified to the level of a toy pistol. When not used, the pistol grip made of playtic may be folded forwards by pressing the button on the left side of the breach, locking both the firing pin and covering the trigger (a system already used on the DM-34 disposable flamethrower). In order to provide more precision, the DM-301 may be attached to a stock which also carries a clamp with a scope on it. Otherwise it uses a simple grenade launcher sight mounted on top of the breech. Since the stock weighs 1 kilogram as well, mounting it drastically reduces the recoil, up to a point where it is lower than the one of a small pistol.

A few projected stats:
total length approx 50 cm
barrel length: 30 cm
barrel diameter 16 mm, muzzle diameter 22 mm (for NATO standard rifle grenades)
weight unloaded 990 gram
muzzle velocity 53 m/s
recoil acceleration 14 m/s² (normal), 7 m/s² (with stock)
unit cost: < 300 euros

Actually, this one was fun to draw, but I might add even more detail or recreate it as a blueprint. :3