That's the M249 SAW version of FNs MINIMI.
It's a gas operated, air cooled, fully automatic light machine gun (based on FNs very popular MAG), firing 5.56x45mm NATO ammunition at around 800rpm cyclic (although 25-100rpm is the realistic rate of fire. Any higher and you'llbe able to read using the glow from the barrel) either in belts or using compatible rifle magazines.
When it works, the short barrelled version is a bit of a bullet-hose, being not particularly accurate and it's very easy to get carried away when firing it at shorter ranges using 100rd bags.
It's an utter bastard to fully take apart for cleaning, especially the regulator which requires 3 hands, a tool and a blood sacrifice before it comes off.
Because of the way the gun is designed, there is a knack to assembling it without looking like a fool. The bolt isn't attached securely to the carrier and there's no audible noise or rotation when the barrel is correctly seated, which makes teaching assembly and barrel changes fun when an inexperienced user places the new barrel on and forgets to give it a shove and shake before dry firing the under spring tension bolt assembly.
Which smacks the barrel off the gun and across the room, closely followed by the bolt.
Other guns (as in, machine guns like the Browning M2 or the FN MAG) usually have a twisting lock or make a set of clicks telling you the barrel is on properly.
That loose bolt can make putting it together a bit of a pain in the ass as well. The bolt carrier assembly on the MAG is very simple, you aim the long cylinder at the round hole and place the bolt onto the rail. Tip the gun nose down, hold the trigger and the bolt will slide home with no problems using gravity.
This weapon requires you to also make sure the rail is aligned perfectly inside the recess on the bolt and carrier. You then tip nose down, hold the trigger and the bolt will fall about a centimetre and then the bolt will have moved slightly and unaligned the carrier with the rail. You then have to reach into the gun, pull the carrier back slightly, wiggle it a little, swear a few times (all whilst holding the trigger) until it lines back up and then it'll allow itself to slide all the way down. Or what might happen is you catch the bolt on the gap in the rail or another random protrusion, which compresses it and promptly falls off the carrier and into the nether regions of space time or hopefully on the floor in front of the gun.
The entire weapon is like that.
The handguard requires the Force to line up to get the pin back in.
There is no set way to get the gas cylinder in. It either will or it won't, and nothing you're doing will influence that.
Best way I can describe it is like a bra. The first few times you're all fumbling, both hands reaching around hoping it's not a fancy attachment or a thread catches and you're contemplating just breaking the bloody thing until she gets bored and does it for you. After a while though, you're pinging it off between thumb and forefinger whilst multi-tasking like a boss.
Also. Never, ever strip this weapon when it's cocked, unless I'm filming you doing it. Cos it's hilarious watching a super compressed spring twat someone in the face.