The BattleMech Manual clearly states that a critical hit to an ammo slot will only explode the total ammunition in this slot (which may be zero, in which case there are no explosion effects).
Does this mean that ammunition is ticked off for each slot individually? Taking the ARC-2R Archer as an example, it has altogether four slots (tons) of LRM ammunition for its two launchers, with 6 shots per slot. Does the player get to chose from which slot the ammo is fired?
That's how we've been playing it for decades, but I'm looking for an official statement.
The counter-argument is that, since ammo of the same type can cross-feed all over the 'Mech, you determine the ammo per slot by dividing all remaining shots by the number of slots. But imho this approach breaks apart when, say, the Archer from the example above is down to only two salvos across its four ammo bins.