The fluff for the Silver Bullet Gauss says the FedCom gave up trying to get a Gauss Rifle to shoot both slugs and cluster rounds and settled for specialized versions firing one or the other. I figure somebody would eventually figure it out, but at a cost in tonnage, heat, and critical space to represent the weapon having to internally reconfigure itself to one mode or the other. So here's my stab at it:
Weapon Type Heat Damage Minimum Short Medium Long Ammo Tons Criticals
Dual-Mode Gauss Rifle DB,C,F,X 2 15/15C 2 1-7 10-15 18-22 8 17 8
Notes: Firing mode must be selected in the End Phase of the turn prior to firing. The entire tonnage of the weapon counts toward Targeting Computer weight but doesn't get the benefit when firing cluster. When it takes a critical hit, the weapon explodes for 20 points of damage.
I figure an LGR would get the same treatment (+1 Heat, +2 tons, +1 crit) for similar Dual-Mode performance.