don't believe it would be likely the other grenades and ammo would explode either.
BUZZ! Sorry, but the ammo would go off. As would the grenades.
A grenade is a powerful enough blast to fragment a grooved steel casing as well as shatter the notched wire inside. It would crush the casings of the bullets, causing the rounds to go off, as well as crack the casings on the other grenades and pressure detonating the blasting cap inside.
BOOM!
As for the "actual" explosion? The concussive force would be pretty serious, and in an enclosed room, the shrapnel bouncing everywhere and richoceting everywhere would tear shit up. The concussive force would be magnified by the enclosed area.
A concussion grenade is basically a grenade simulator, which is basically a grenade without a steel case (cardboard) and no notched wire interior lining (instead more BANG! fun)
Let me think...
OK, you can fuck shit up with a grenade tossed into a room, so we say six grenades going off at once, that's going to be... crap... around 15 lbs net explosive weight. The bullet casings have a good chance of being turned to shrapnel, and the bullets
might cook off in random directions. A bullet going off underfoot without a chamber will still blow a hole in your foot.
I'd just say... per 10 bullets, add 1d4, per extra grenade, add an extra 2d6 to the damage. Per grenade extend out the "damage radius" (God I hate how grenades were done in d20 Modern) by 5'
Should work for quick and dirty.
I'm thinking of an accident that I heard of, where a bullet struck a grenade on someone's harness. Yeah, it blew him pretty much in half, and the explosive deonated the ammunition as well as the grenade on the other side of the ammo pouch. The grenades and ammunition on the other side did not sympathetically detonate though.