Nope, the ammo mass is entirely ammo or nearly so. Any of the feed mechanism is accounted for in the weapon's mass.
8 shots per ton is a balance mechanism. There is some give on the construction statistics.
Take Arrow IV for example, its a 15 ton launcher plus 5 shots per ton, yet normally depicted as a 5 shot 16 ton missile launcher, rarely with more ammo than that. Most set ups don't even have a feed mechanism, just five tubes, all full.
Arrow technology possibly varies, the Capcon uses over big rockets on a simpler firing rail in the Catapult, Clan Wolf might use more compact rockets in a more complex mechanism in the Naga. It amalgamates into the same thing.
Or how about vehicle design, the engine mass is for the engine, foot down on canon there, and the transmission is made of thin air. The additional mass for tracks is calculated indirectly through modifiers to engine mass in suspension levels, and this value is further diluted if the vehicle has a fusion engine, which presumably doesnt effect transmission mass much. Consequently it is wrong to take anything other than a holistic approach.
Back to the gauss rifle for a further explanation. If a guass rifle is arm mounted with torso mounted ammo the ammo feed will weigh more and take more space than if the ammo magazine was in the arm adjacent to the gauss rifle. however this mass is amalgamated and simplified and it works as is. By it also means that you have to accept that the mass of the weapon and its ammo is the mass of the weapon and its ammo and it might not be possible to subdivide this down. With so many variables to add we cannot just take the mass of a ton of gauss ammo and divide it by eight to get an estimate of shot mass. Place that ammo two locations away from the weapon and it might take a sizable proportion of the tons allocation for additional feed mechanism to get there.
If this is not enough consider a targeting computer. Why would a targeting computer for a gauss rifle weigh more than one for a small laser, its software right? On a computer board in the main control hardware. targeting computer might cost a lot but its mass is negligible, however the targeting computers mass comes from the fine control waldos it uses to auto correct aim for you which will weigh more to zero in a gauss rifle for target assist more than it would for a small laser. We could accurately consider a targeting computer as a 0 mass 0 critical slot component in the head, but with additional mass in fine adjustment waldos all over the mech, but we call it all 'targeting computer'.