Personly i think its just a hint at the old shouldermounted wepons from old times. On the other thing the plasmacannon is a single shot gun, like the lascannon and the rocket launcher, so yu wouldnt suffer consistant recoil like with a mashine gun. So it would make them easier to aim for that single shot.
Wh40K and weight? You all forgot the anti-grav suspensors. This would look nice as HI, with suits that are really for normal and not stick persons.. until I noticed other proportion issues. GW as always...
I can imagine that, with several tens of thousands of years of technological advancement, firing something from the shoulder isn't really, you know, the biggest suspension of disbelief required.
Single shot sniper from the shoulder is one thing, but with something that could be doing 600rpm that's another story entirely. Which is a moot point anyway as I don't think any of the heavy stubbers in Necromunda (the weapons I was talking about in particular) are shoulder-mounted; they're all hand-held and mostly look like they're meant for stands/pintles. I'm actually not sure if any heavy stubbers were shoulder-mounted in 40k, either. And technology is a funny thing within the Imperium - it really has a hitech/lowtech kinda vibe. A lot of advancements made; a lot of prior advancements forgotten.
It's funny, but the smartgun rig from Aliens is actually a modified Steadycam setup! If you put a camera down on the gun you could field that today... That's weird... I don't remember seeing a small heavy bolter, and I had some of the original RT metals. The Scout heavy bolter from Advanced Space Crusade was bigger and had a recoilless vent out the back, so it looked better balanced. But then the heavy bolters got really big in 2e (up on the shoulder) and then especially in 3e (hand-carried). Gotta admit that I prefer the 3e (and 4e plastics), particularly the lascannon. Just a bigass sniper rifle.
There may not have been as many as I thought, but they were back when the weapons were sculpted on and before they just made them all shoulder-mounted, presumably so they could be stuck anywhere. There was a hand-carried heavy bolter in Space Crusade as well.
The very first heavy bolter marine was that one and one of the first lascannon marines looked to be carrying an Aliens smartgun, but it was followed less than a year later by the newer Marine HW versions with shoulder mounted/carried weapons. Heavy bolter, Heavy Plasma Gun, conversion beamer, lascannon and heavy grenade launcher. From the 1988 catalog. Guard used the same weapons - they had heavy troopers moulded to take the guns.
