Gone to Ground has a great post that's a few months old now about converting Imperial Guard Autocannons into Rifleman Dreadnought arms.
The funny thing is, I used IG Autocannons also, but my process was MUCH harder and, in the end, doesn't look any better, lol.
This is an elegant and cheap way to do it. Check it out.
Oh, and added to the blog roll. ;D