With regard to the barrel, I have heard of guys cutting the handle off a short M249 barrel and using it in their Mk46. It's a bit heavier and you get the advantage of a muzzle brake on the Para barrels. (Talking about real guns, here).
I'll look to see if I have any pictures of the fluting on my Classic Army LMG barrel, but I doubt I do.
If I were to do it again, I'd use a Para barrel and put the rail "upside down" like on the MOD 1's.
I cut off/milled out alot of stuff on my LMG, and cut off quite a bit of weight! It was lighter, and balanced better.
I also removed the Para stock, and made a collapsible stock. This was another thing I took from guys running the real Mk46, and it shaved a few pounds off the overall weight of the gun.