Ever since someone decided to introduce the Bulging Biceps talent, we've had people running around toting heavy weaponry all on their own and it kind of breaks the balance between sizes and types of weapons.
Blame Andy Chambers and Jervis Johnson.
Bulging Biceps originally appeared in Necromunda, as a Strength skill allowing characters to move and fire Heavy Weaponry. All Heavies in Necromunda carried those heavy weapons (and demonstrably, ammunition for them) by themselves - the heavy stubber was the common starting option, but lascannons, heavy plasma guns, heavy bolters and autocannons were all available, and were all treated identically in regards to how they were carried.
Being a part of 40k tradition, the talent made its way into Dark Heresy, as a way for strong characters to employ heavy weaponry without bracing. This covers characters like those Necromunda Heavies, like "Try-Again" Bragg from the Gaunt's Ghosts novels, Ox from the Last Chancers, Gunnery Sergeant Harker of the Catachan Devils… the list goes on. The talent's innate prerequisites are such that an average starting human (Str31) needs three Strength advances to meet the prerequisites for it - even for characters with cheap strength, that's still 850xp. That shifts with characters that start out strong - Feral Worlders, lucky rolls, etc, but that's not without justification.
If anything, the factor that has made the talent too easy to obtain is point-buy characteristic generation - being able to freely pick and choose your starting characteristics can (and often does) make a mockery of the normal talent prerequisites.
Bulging Biceps exists to model a particular kind of character that has long existed in 40k. Its absence would make creating those sorts of characters impossible.
Weapon teams in the Imperial Guard (which are, aside from Eldar Guardian squads, the only instance of weapon teams in 40k), from the perspective of the RPG, are a matter of convenience - one guy carries the gun, the other carries the ammo (and loads the gun - cuts down on reload times) - and they aren't assumed to be sufficiently brawny to have Bulging Biceps (hence the tripods - they need to Brace).
[/QUOTE efidm=718754]The autocannon is as powerful as it should be, and quite versatile - there is a reason the Astartes used them as their squad support weapons of choice during the crusades, and it was only the introduction of the assault cannon and man-portable multi-meltas, lascannon etc that saw their decline.
One thing worth noting - the Assault Cannon in Deathwatch is a "Mounted" weapon, and thus can only be used on vehicles or Terminator Armour.
Personally, my preference when it comes to sorting out the matter of bracing and heavy weapons is the following (and I've suggested it to no avail during work on the BC and OW rules): Bracing a weapon adds +20 to hit when firing it. A character with Bulging Biceps can (RAW) fire a heavy weapon without the normal penalties for not bracing, while Suspensors count a weapon as Braced… which is an important difference when Bracing does more than remove penalties. At this point, characters using heavy weapons are encouraged to Brace when possible, as it makes them more accurate, but characters with Bulging Biceps can still hip-fire heavy bolters if the situation demands it. It also makes the difference between Bulging Biceps and Autostabilised matter again.