This has probably been done before, but here is an easy way to build a believable (I think) meltagun out of spare parts from mostly-useless Death Company bits.
What you'll need:
1. A bolter -- almost any bolter will do, but I need the Death Company bolt guns like I need a root canal, and there's 5 in the pack.
2. An inferno pistol. Some kits you get them from include Death Company, Sanguinary Guard and Blood Angels Tactical Marines.
A picture is worth a thousand words, so, here you go:
Essentially, I cut off the front of the Inferno pistol, starting right behind the first bottom ridge. Without anything else, that's the front of the meltagun. On the gun end, I cut off everything to the right of the barrel, and the targeting reticle. The two pieces fit perfectly, and if you allow a small gap, you can have a pleasing vertical gap that will match the original horizontal gap on the bolter. On all of the
DC bolters, you will get to preserve the Blood Angels/Death Company iconography on the botler, as a bonus!
Then, you need to shave off the bolter magazine, cutting first to simply remove the clip, and then cutting closer, near to the trigger so that the drop is not as great. II notched it 45 degrees at the end of the block, to soften the squareness of it all, but this isn't necessary. I then cut the cylindrical device on the top right of the inferno pistol (inferno fuel cell?), cut off the little ammo lever on the bolter, and glued the fuel cell to cover it.
I used a little bit of modelling putty to fill up the original horizontal line in the inferno pistol, because it doesn't line up. I also used some putty on the top (you can't see this), and then a hobby knife to score it, to make the ring around the front part of the gun consistent and cleanly aligned.
Finally, I used a small, ridged hose to connect where the bolter clip used to sit, to near the start of the gun. I don't think this is, strictly speaking necessary, since combi-meltas don't have such a cable. However, I had spares from the left hand supporting the
DC Thunderhammer, because (a) I don't think I'll ever use a thunder hammer on a
DC and (b) even if I did, the power cable isn't exactly a must have. I think I'd put a bolt pistol in the left hand anyhow, as I typically hate crossed arms. Speaking of which, many of the converted meltaguns can be posed with a 1-handed meltagun, freeing up the other hand to display a chainsword -- again, the main reason being, this allows you to paint the chest area with the model built.
I hope you like this! Please let men know what you think of the finished meltagun

Is it good enough to run
WYSIWYG in one of your games?
Edit -- Oh yeah, Infernus, inferno, potatus, potato

I always mix up what I call them and I didn't feel like going into the graphics file and correcting that

I think they are all called Inferno pistols now?