Exactly. If you don't have CASE (either because it's not available or you don't want to dedicate the spare tonnage because you don't have a lot of ammo on the 'Mech to begin with), you
need to pad the location the ammo is stored in otherwise it's even easier for it to go off. And on most 'Mechs the Center Torso is the best place to store that ammo. It's got 10 out of 12 slots already taken up, and while a critical hit to the engine or the gyro sucks, it sucks a lot less than an ammo explosion.
Gezkill's crit was a... I was going to say "one in a million" shot, but let's calculate the actual probabilities. Let's be incredibly average and assume it was a medium-range shot with an average pilot, on the move, against a walking Warhammer. That would be 4 (gunnery skill) + 2 (range) + 1 (target moved 4 Hexes) + 1 (attacker walked). That's a to-hit number of 8+, or a 41.66% chance to hit.
Once hit, you'd need a 2 to get a through armor CT critical. That's a 2.77% chance, on top of the original 41.66%. Then you need to roll 4-6 (50%) to hit the second block of criticals. And then an exact 5 on 1d6 (rerolling 6s, so effectively a 20% chance). If I have my numbers right, that's a
0.11% chance of that shot hitting MG ammo bin and wrecking the 'Mech. That's about 1 shot in 800. The first variable (the to-hit number) may change, but the other probabilities (2.77% CT through armor crit, 50% section, 20% location) don't change. So, in the vast majority of situations, you're far more likely to have had the 'Mech shot out from under you than you are to actually be blown up by a CT ammo bin.
(For the record, the chance of the same shot blowing up the ammo on the right torso (assuming a right-side shot) is 0.23%.)
But just in case you want to be safe and want to disable the Fusion Engine Ejection System on your Warhammer, you can always dump ammo on your first turn.
View attachment 2396751View attachment 2396752
Total Warfare, p.104
Or, if you're playing with friends who aren't rules-lawyering jerks, you can just say your Warhammer is coming into the field with an empty MG ammo bin. Or with X amount of ammo. Partially loading ammo is a time-honored tradition in armored combat.