I really dig that rule. 1t of Machine gun is pretty ridiculous, so just rolling up gun and ammo into one sounds reasonable.
An M2 weighs 40 kgs. Whatever the fuck are machineguns in the 31st century made of to weight a full ton? Even if it's several MGs duct-taped together, that would require more than 2 dozen M2s jammed in one crit to get even close to a ton...
In fairness, you could likely just completely ignore ammo tracking for MGs, make MG ammo bins a 5-point internal explosion and have an even simpler solution to the problem. Just
assume there is ammo in the bins, but that it's not made of high explosives. If all MG ammo bins are destroyed (or detonated), then the MGs are assumed to be out of ammo. Done.
Really, there is literally no vehicle or 'Mech in BattleTech that's likely to run out of machinegun ammo within a reasonable length combat. Even the Piranha, Patron Saint of Memetic MG Builds, only runs out of ammo after over
fifteen rounds of constant fire. That's literally over 2.5 minutes of constant bullet hosing before the two half-ton bins are empty. Likewise, the Timber Wolf Prime is an infamous "we'll just throw a couple MGs in there as an afterthought" design, and even with
two of the damn things it takes
sixteen and a half minutes to run out of ammo. That's longer than your average Clan-on-Clan engagement. The pilot could tape down the "fire MGs" trigger at the start and still have ammo left over by the time he ran out of targets. That's why the rapid-fire rules for MGs were released. And yet, keeping track of all that ammo is a hassle. SRM-2 and AC/2 ammo coming in lots of 50 and 45 are bad enough. 200 or even 100 rounds per bin, with most designs carrying only one or two MGs, is just too much work to track for no benefit.
And if someone tries to abuse that house rule by bringing in a MG boat with a single half-ton of MG ammo and literally every single remaining crit slot packed with a machinegun, you first wreck that 'Mech before it can come into range with almost literally
any other weapon in the game, then slap them upside the head and tell them to fuck off for wasting everybody's time rolling so many attacks per shooting phase.