Sometimes you get a model that looks better in person - unless I got a lightbox and a better camera this gunner is just going to look A Little Weird. Notes on the model are under the cut, but first I want to point out the barrel of the meltagun - it's my first attempt at adding a kind of color transition! It's not great, but it's pronounced enough to make the gun look especially dangerous when fired.
The meltagun doesn't have a clear military analogue. As far as I can tell, it's a close-range heat dispersal system: when the soldier pulls the trigger, the gun superheats all matter in front of it, turning even stone and metal into sludge. In Relic's 2011 Warhammer 40,000: Space Marine, the meltagun was a kind of shotgun that launched superheated particulate to devastating effect. Lexicanum suggests that melta weapons work through either subatomic agitation of the target or a fusion reaction. Whatever the case, it isn't unique to the Imperium - the T'au and the Eldar use meltas extensively in anti-armor operations.
In Kill Team, the melta gun is the strongest weapon in the Veteran Guard's arsenal: dealing a hellacious 6 wounds per hit and boasting AP2 without the need for a critical, it's the first choice for dealing with armored targets and enemy commanders. On a crit the weapon deals four mortal wounds in addition to three standard wounds, bypassing the more esoteric defenses common to elite factions. Anyone running Marines, loyalist or heretic, will give the melta gunner a wide berth. The only weakness is range - it's only a 6" gun, so make your shots count. A good bet is to use the Combined Arms ploy, so when a trooper takes a potshot at a VIP the gunner will get some precious rerolls.