Summarized Chime-In..
IMTU:
Gauss Rifles and High-End slug throwers all have a power cell (located inside the stock). This is used to power the electronics/sights/stablization etc.
It's rechargeable, either from normal power sources or powered armor.
Energy for shooting a gauss weapon is in a magazine battery (IAW canon). Although I allow supplemental energy to be provided by the weapon or armor if needed.
So, while the power source to fire the gauss rounds are in the magazine...if that's depleted, you can still draw from the weapon (draining it much quicker) or armor (if powered). It seems the common-sense kind of back-up the military would have designed, to avoid the 'dead magazine' scenario.
NOTE:
I do NOT mean to imply that powered armor is needed for gauss weapons. Just that if you have it, you can use its power for your weapon in an emergency.
Also, for my purposes, Combat Environ Suits and Combat Armor have (tappable) power sources as well (which exist to supply power for Hud, commo gear, monitoring of the life support, etc).