I encountered the idea of cut-down M249s on a cyberpunk website. It is a fun idea for TS if you want to give criminals or the low-strata of society an unanticipated offensive capability!
Booster is one of the street names for a weapon that has become increasingly available in certain urban areas. Initially seen in the United States these weapons have also been encountered to a lesser degree in other continents. It seems likely that the first weapons were from a large number of old M249 Squad Automatics pre-dating the US military's switch to caseless ammo. An alternate theory is that the weapons originated from copies of the Minimi produced by an Asian nation. The booster variant has had its stock removed and barrel cut down to the minimum possible length, allowing it to be hidden under a coat. These modifications do have an effect on the accuracy and reliability of the weapons. The user may carry a bag with 300 to 600 rounds of belted ammo and/or have an accomplice carrying additional belts. “New” examples created by minifacturing have been encountered for several years now and it is probable that the compatible ammunition in use is produced in the same way.
GUNS (LMG) (DX-4 or most other Guns at -2)
|Minimi Booster 5.56x45mm||4d pi||3||420/2,600||17/3.4||12!||100(5)||
 Can use either a 200-round disintegrating belt (6.4 lbs.) held in a 0.6-lb. plastic can (which rattles: -1 to Stealth) or a 100-round belt (3.2 lbs.) in a 0.2-lb. soft pouch. Requires metal-cased ammunition. Cannot use 5.6mm caseless rounds.
 Can use M16 rifle magazines increasing RoF to 15! and reducing Malf. to 15. Such magazines are no longer common and if desired would need to be especially minifactured or 3D printed.
 The short barrel results in excessive muzzle blast (+2 to Hearing and Vision rolls to locate the weapon firing in the dark).