Wouldn't ya know it, there are rules for exactly this sort of situation! I present, the Sympathetic Detonation rules from p. 90 of Arsenal.
QUOTE (Arsenal @ p. 90)
When an unprotected explosive charge is struck by the blast of another explosion, roll the basic rating of the explosive that gets struck (unadjusted for the number of kilograms involved). The threshold for this test is 10 minus the remaining DV of the explosion, with a minimum of 1. When the explosives are enclosed or protected somehow—even by being under a character’s armor jacket—add the Armor rating (or Impact armor rating ) to the threshold. If the test rolls any net hits, the explosives detonate immediately.
For purposes of sympathetic detonation, commercial hand grenades contain Rating 8 explosives, their plastic outer shell giving them Armor Rating 2. Mini-grenades, rockets, and missiles use Rating 12 explosives, with an Armor rating of 1. When a character carries multiple grenades, consider all of them a single charge in order to reduce the number of dice rolls. If these accidentally detonate, the DV is equal to that of the most powerful grenade carried, with a +1 modifier for each grenade after the first, regardless of its type.
When two or more explosives are in physical contact, they are considered a single charge, and the above rules are not needed. If there is a gap of more than a few centimeters between them, they should be seen as separate charges.
So first step is to calculate the effects of the actual Self Destruct.
QUOTE (Arsenal @ p. 149)
The "explosive self destruct" rigs the weapon with explosives that will not only destroy the weapon, but also cause 10P(f ) blast damage with an AP value of +5 (AP –4 for the person holding the weapon) and a Blast value of –2/m.
So first the wielder needs to stage down 10P -4AP.
Then, the mini-grenades inside the launcher suffer the same. They are treated as Rating 12 explosives with an Armor of 1, so you roll 12 dice to see if they go off. The threshold is equal to 10 minus the remaining DV of the initial blast. In this case, since the mini-grenades have their armor negated by the AP of the self destruct and cannot resist the DV of the blast, the Threshold is 10 - 10, meaning it would be 0 except for the minimum threshold of 1.
So you roll your 12 dice, and most likely get more than 1 hit, setting off the mini-grenades. I'd say the casings of the mini-grenades count for keeping each explosive "separate", and I'd treat them as separate charges. They all explode together, creating a blast equal to the DV of the most powerful mini-grenade among them, +1DV for every additional mini-grenade.
So for an ArmTech MGL-12 loaded with a dozen High Explosive mini-grenades, we're talking 21P, –2AP, with a blast falloff of –2/m, meaning a 10m radius area of effect, or a 20m diameter.
The poor bastard who triggered the Self Destruct has to resist a grand total of 31P (not accounting for AP, which would effectively add another 2DV), and his buddies are going to be lucky to escape with minor bangs and bruises if they're fortunate enough to be 30 or so feet away when he blows.
Alternately, if you or your GM disagree and feel that all the grenades should
not count as separate charges, then the second blast is simply equal to a standard HE grenade going off - which doesn't make much sense, but oh well.
~Umi