There is a distinct lack of Firefly in this thread.
Allow me to correct that.M-815-12C Marlin IV
Type: Lever-action rifle
Origin: Barrett-Winters FMC
Calibre: 6.5x35 Barrett-Winters HMR
Safety: Safe/Fire
Sights: 2:1 pillar iron sights (ESI "Leo" VRMS fitted over rear sights)
Based on a modernized Marlin lever-action rifle, the M-815-12C is popular with sport shooters and free traders everywhere. With its unique integrated four-tube manually-revolving magazine, it is well-suited to tight quarters; hunters can reload without moving their aim or disturbing their cover, and shipboard users can reload from pockets or ammo belts, rather than bothering with bulky detachable magazines.
About the magazine:
The magazine consists of four tubes, which revolve on a ratchet. To load it, you feed in 5 rounds to a tube via the (left-side on this right-handed example) feeding port, operate the lever with the safety on, which brings round the next tube, and repeat. Once all tubes are loaded, operate the lever once with the safety off. This chambers one round, and brings the partially-empty tube to the feed port for topping up.
Credit to Ulfr and Ford for helping me make this NotCrap™ o7