Being a large caliber rife with explosive rounds, the discharge of this weapon will certainly be something to glorify. In terms of the rate of fire, it is to my understanding that the Warhammer 40k Space marine game really nailed it rather well. But the firing sound in my opinion was really well displayed in the Ulramarines movie. (though the movie itself was not too impressive) So what do you guys think?

A mixture between Dawn of War 2 and Space Marine imo would be a pretty good sound. The only problem I really had with the bolter in Space Marine was that it didn't feel like it had an impact, where the bolt pistol sounded devastating and weighted.

I remember reading in a book when a Guardsman described the sound of a bolter as being "roaring thunder" or something similar. As long as it has a pretty bass-y sound with a crack as it leaves the barrel, it'll be a-okay.

I tried doing something like that for the heavy bolter, just need a better weapon of an higher calibre then a machine-gun.

https://soundcloud.com/legendary-18/space-marine-pissed-off-mp3

Yeah I would have to agree. Imo the boltgun here matches a "roaring thunder" sound rather well:
View: http://youtu.be/tsvCLG1QZjs

I feel that this is not payed attention to enough in all the 40k games that have been. Ian Watsons Inquisition War trilogy touched upon the subject, explaining that the barrel was fluted to make a truely unique sound. I belive the word he created was RORNK. Though I really didn't care for the books, I would love to hear someones interpretation of this sound.

maybe we should just make a bolter sized round and fire it and see for our self's what it sounds like

The bolter is not a rifle. It's basically a mini missile launcher. I always thought it should sound like an Stinger missile launcher.

The bolter is very much a rifle. It has rifling, and fires just like a rifle. Just happens to be explosive rounds that exit the barrel.

It doesn't fire just like a rifle! "A bolt is self-propelled; it features its own integrated solid propellant that propels the bolt at high speeds, essentially acting like a miniature rocket. The propellant itself is shaped to control the bolt's direction and speed; however, this method of rocket propulsion would normally warp the barrel due to gas pressure"

Just a little after that: "As well as the rocket propellant, a tiny amount of conventional charge is also utilised. This charge is just strong enough to force the bolt out of the barrel and ignite the bolt's propellant."
It doesn't rely on gunpowder exploding to drive the round, but it relies on it to get the round out of the barrel, otherwise the rocket would fill the chamber with rocket propellant, making it explosive.