I'm not sure. Is there a way a boltgun is suppose to sound? It's varied from Dawn of War, Space Marine, and even the Ultramarine movie.
As Schaff-Ili and Freke say above, Bolt shells are a composite munition that utilize multiple charges at different points of their firing. 1: Clearing Charge. This is the initial boost used to clear the barrel of the weapon. The reason for its existence is twofold, one, to keep the recoil of such large caliber weapons (.75+) manageable, and two, to avoid overpressurisation in the barrel/chamber. This charge fires like a conventional round. 2: Propellant Charge. This is essentially a solid fuel rocket that drives the bolt toward its target. This charge can vary in both composition (stalker shells use compressed gas to reduce noise) and velocity (pre-heresy there were at least super and sub-sonic variants, each more effective against different foes, now there are even more). 3: Mass Reactive Warhead. This is the painful end of the shell and the composition can vary enormously. As there are 3 distinct charges it then stand to reason that there should be 3 distinct noises that feed into each other. Clearing would sound like a short firing, large caliber rifle (short because of the reduction of charge as it doesn't need to propel the shell the full distance). The propellant for a conventional bolt would sound like a small solid fuel rocket at high velocity (note that this and the warhead could vary a lot depending on shell type but lets stick with conventional for now.) The warhead would detonate less than half a second after impact but that pause is important (they are mass reactive for a reason). A small charge of C4 would serve as a suitable proxy. In summary: 3 noises (Large Caliber Rifle, Solid Fuel Rocket, C4). Now take that and put it on full auto. There's your roar.
It should sound like the holy hand grenade in worms to the space marine firing it but to everyone else it should sound like just a roll of thunder.
Anyone heard the Munitorum Volume I bolter sound? Well that bolter sound but with a bit more base is how I like it! The Vulcan Mega bolter sound is quality!
With all respect, but - they ARE automatic even in this movie. At least there are lots of 2-3 round bursts. Why no longer bursts? There's merely no need in it - due to the destructive power of the bolter itself and more than decent training of the SM.
