They are described in various codex's and rulebooks as "heat rays" which would suggest something other than a single blast. The term ray would imply that its not like a lascannon which is a one shot blast of energy.
the melta gun AKA Melter, cooker or vape gun works by sub-moleculer thermal agitation in a manner comparable to microwave irradiation
And as we all know, microwaving something takes a little time, so the comparison would imply that it does not do it in one blow, but takes time to "cook" the target.
Additionally several of the flavour texts from both 2nd, 3rd and 4th ed rulebooks describe the hissing noise as the weapon melts its way thorugh objects, implying that it requires to be fired at objects for some time if the armour is thick enough, thus the idea that its not a single shot weapon and more of a directed beam.
The only definative piece of artwork showing a melta being used is on the box of "assault on black reach", where in the background of the image a dreadnaught is shown firing a stream of red hot something from what is clearly a multimelta.
http://goblinminimart.com/wp-content/uploads/2008/12/aobr-front.jpg (directly behidn the knives on the orks back)
It looks like a hyped up, more precise flamethrower.
Theres also a image of a dark angel firing the hand held one somewhere, it looks like an afterburner with a beam coming from it.
So, we belive that as theres not much fact, that its a beam weapon.
