As far as I know, the way things currently are, moving towards a square occupied by an enemy with a gun equipped rolls for a fist attack, and if you score a kill, it's a kill for the gun.
Why not have fun with this?
Guns like pistols, shotguns, and rifles can all have close combat applications, and of course a Marine would be familiar with them - pistol-whipping, butt strikes and bayonets, to name a few examples. Why not give guns their own melee dice - nothing major, just more than 1d2+SoB/Brute. Combat knives could possibly be used to mod weapons (probably granting slightly less than straight-up knife damage). More damaging combinations and different dtypes could be applied in the event that something more exotic is assembled, like some kind of chainsaw gun laser bayonet.
Pistols - pistol whips would probably be fairly fast but do little damage.
Shotguns - normal speed and slightly more damage.
Rocket Launcher - these would probably pack the most heft.
Rapid weapons - depends. There are multiple types - smaller, handheld ones could be used similar to shotguns, and there are bayonet possibilities.
Heavier options - assault cannons, BFGs and miniguns may not be as useful in melee, so maybe default to fist or no melee attack.
It would be sort of neat if kills scored by gun-melee were counted separately, but eh.
This may also open up some fun chainsaw gun assembly possibilities. Not sure how all of this should interact with Berserker.
Anyone else have any thoughts?