Once upon a time, many moons ago, I bought some Autocannon dudes from Anvil Industries. The guy who runs the place makes some really nice models with almost no clean up required. If you're interested, check it out. He sells tons of specialty weapons for Marines, as well as the Black Ops bodies...
This is what they look like straight from Anvil Industries.
I was looking for a little more of a GW aesthetic, so I replaced the helmets with some Grey Knights heads, swapped out the shoulderpads and put some regular backpacks on. I think the effect works. You'll notice that in the picture below, I have Chaos Marines backpacks on. I changed my mind and went with regular loyalist backpacks.
In this shot, you can see the backs of the legs. They have cabling running up the backs of the calves. I like the idea for the heavy weapons. I like to think that they're extra power assist systems to handle the recoil of an Autocannon.