Go for the double heresy by using it as a melta-bomb.

The plasma gun overheat discharges through the melta gun mech, meaning that rolling a 1 actually gives you an extra shot.

@Slouping  
Pretty much. There’s armour saves which are granted by your armour, which you can take to shrug off a wound unless the thing hitting you has a good enough AP value to ignore said save. Then there’s invulnerable saves, which you can take in place of an armour save if you have it and your armour is penetrated, or if your invuln is just better. You can take those unless you’re hit with one of the like three weapons that specifically ignore them, or if you’re hit by a D weapon and it rolls a 6 (yeah just…ignore that, for now). Then there’s Feel No Pain, which isn’t TECHNICALLY a saving throw but still functions like one and anyway long story short GW wants you to buy the plastic men with better guns so they can kill your opponent’s plastic men without getting a decent save. As for Gets Hot, that’s when your guy essentially ends up burning and potentially killing himself with his own plasma gun, though normally you get to use any saves you have if it happens.
 
Hey, just because it’s a ranged weapon doesn’t mean you can’t bludgeon a guy with it. Or toss it at your enemies when it’s about to explode.

@Kharn The Generally Swell Guy  
Never really understood that Save mechanics, but I think that means it’s basically a “go f*ck yourself” to the ennemy because he can’t roll his save, right?  
I mean I’m more of a Fluff than the Crunch guy. I made this gun from how both weapons work in the Space Marine Game (and it’s a game which capture how most weapons feel just right, unless for the power, I mean PLASMA weaponery.)
 
Also to get one? Succeed to persuade a Tech Priest it’s not heresy or an Heretek (the heretek might be more inclined, but chaos weapons are, well, chaos weapons) But since you’re called Kharn/A really Nice Guy, I don’t think you need something ranged :3

This weapon just screams “Gets Hot with no saves allowed”.
 
Where do I get one?

@Sanguine_paladin  
I’d say the Melta is the One shot.

I wasn’t expecting that one to get on Derpibooru. oh well..

And which one is the one shot weapon? Plasma or melta?

Great.
 
One shot that isn’t likely to fry the user.  
(I have a bad track record with plasma weapons in game)