[Bradd]

For what it's worth, I assume that grenade damage already accounts for damage reflected from the ground (because it's almost always there, and generally very difficult to destroy). I do count bounces from the ceiling though, and back off the ground if they make it that far. Likewise I ignore non-orthogonal reflections, so I count a total of five shock waves, one in each cardinal direction and one upward.

Given what I know about flash-bang grenades, I don't think they should accumulate damage like that. They're designed to stun by messing with your senses, rather than doing serious damage. Realistically it'd be worse in a tight space, but it probably shouldn't add up damage effectively in game terms.

Then again, damage probably shouldn't add up as effectively as it does for any explosion. Even if you don't breach a wall, a lot of force and shrapnel gets transferred into it. You don't need to put a hole clear through to soak up the dangerous bits. It's another reason I think you should use the overlapping explosions rule (secondary shockwaves deal half DV).

[Chaemera]

Halving secondary wave damage is not an unreasonable method if you feel that the rules are too harsh. The underlying problem which you touch on (transfer of force even when the walls stay standing) is too complex for any simple solution if you want to be good to realism.

The reason being is that it depends on both the type of explosion and the walls its impinging against. For example, Imagine a theoretical vulcanized rubber-walled (say, 20 cm thick) room. Detonate a frag grenade inside. What happens? All those rebounds SR4 calls for never happen, because the shrapnel sticks in the rubber. Now, detonate an HE grenade. Guess what? You're dead, very dead, because the vulcanized rubber is stiff enough to reflect the overwhelming amount of the shockwave back towards the center of the room.

Now take a thin-walled (5 cm) steel shipping container. The result is the exact opposite. The HE grenade will bow the walls of the shipping container, transferring its energy away from the center point, but the shrapnel will bounce around in there until it finds something fleshy or it's shed all it's momentum (which'll be a while as far as anyone inside is concerned).

I could go on all day with examples where different explosions, given a different barrier, will result in a completely different scenario. So, they simplified it, if the wall withstands the blast, the wave comes back at ya. If it doesn't, the wave leaves you alone.

If you want to be nicer without just assuming half the second (third, fourth, etc) wave's damage on impact, you could use your GM magic to apply a "most rational" solution case-by-case. Perhaps, since they're in an asylum with padded walls, you ignore the frag grenade's rebound entirely. And when the HE grenade's explosion bounces off, you figure there's enough give in the padding to reduce the wave's explosive potential by, say, half structure?

I find these "as you go" type rulings (if your players are comfortable with it) are a far cry better at establishing a sense of realism without completely negating the fear of chunky salsa than any static rule could be. After all, that tactical consideration should be on their mind when they decide to take cover in a shipping container. It may even coax them into a sub-optimal solution, using wooden shipping crates as cover and spreading out. Heck, that's the point of grenades, to keep people from digging in too deep to dig back out.

[Bradd]

Just to clarify, I was thinking of people taking cover behind shipping containers, not inside of them – behind the corner of a building is another good place to get fragged, and yet a smart place to be when bullets are flying. Anyway, your basic point is valid: Grenades flush people out of hard cover (or send them in deeper, so they're no longer a threat). I would simply prefer that they were better at doing that, and not so good at killing people outright in that situation.

By the way, another thing that's been bothering me is the "rebound repeatedly off each of the six surfaces" rule, not just because it's unsurvivable, but because "six surfaces" is a really naïve way of describing reflection mechanics. What happens in an octagonal room, for example? While the shape of the room does influence reflections, it really shouldn't matter much when it comes to the overall health hazard. I've been thinking, it might make sense to just use the single most damaging reflection path. That would also mitigate some of the realism and overkill concerns that I have.

Paradox: Grenade goes off in a narrow corridor. It's not strong enough to blow out the walls directly, but after a couple of rebounds the DV is high enough. However, if it blows out the walls, then you don't get a couple of rebounds, so it's not strong enough to blow them out.

[Chaemera]

Just to clarify, I was thinking of people taking cover behind shipping containers, not inside of them – behind the corner of a building is another good place to get fragged, and yet a smart place to be when bullets are flying. Anyway, your basic point is valid: Grenades flush people out of hard cover (or send them in deeper, so they're no longer a threat). I would simply prefer that they were better at doing that, and not so good at killing people outright in that situation.

That's why Shadowrun has the Hand of God? 

By the way, another thing that's been bothering me is the "rebound repeatedly off each of the six surfaces" rule, not just because it's unsurvivable, but because "six surfaces" is a really naïve way of describing reflection mechanics. What happens in an octagonal room, for example? While the shape of the room does influence reflections, it really shouldn't matter much when it comes to the overall health hazard. I've been thinking, it might make sense to just use the single most damaging reflection path. That would also mitigate some of the realism and overkill concerns that I have.

Hey, just 'cause some of us consider the reflection paths off of the heavy steel benches that were welded to the floor of the shipping container (so the cargo had somewhere to sit), doesn't mean they had to be too complicated in their thinking.

To be fair, the general rule says "If the wallss or doors hold up, the blast is channeled" and then tell you how to handle channeling (ie, the wave comes back at you). The only time they specifically talk about 6 surfaces is when describing chunky salsa. Of course, the fact that they say 6 suggests that you're supposed to include waves reflected off the ground. Better hope you've got a high enough DV to destroy the planet...

Paradox: Grenade goes off in a narrow corridor. It's not strong enough to blow out the walls directly, but after a couple of rebounds the DV is high enough. However, if it blows out the walls, then you don't get a couple of rebounds, so it's not strong enough to blow them out.

Eh, it blows out the walls and deals damage to the characters as if it hadn't been staged up?

[Wayfinder]

Paradox: Grenade goes off in a narrow corridor. It's not strong enough to blow out the walls directly, but after a couple of rebounds the DV is high enough. However, if it blows out the walls, then you don't get a couple of rebounds, so it's not strong enough to blow them out.

I've always ruled that the damage stacks until it blows out the wall.

As for the idea of reflecting off the ground, I've assumed if the grenade lands on the floor it doesn't reflect unless the shockwave rebounds off the ceiling to the floor and back up. however say a grenade goes off in your pocket (happened once) then the ground would reflect the shockwave.

Speaking as someone that routinely makes and breaks walls, most residential and even commercial structures under 5 stories are not using what SR calls structural material. Now a high rise or large span like a hotel ballroom would. If your wondering if a HE grenade would punch out a typical wall, the only tool I use to remove sheetrock/concrete board is my hands.

[Kontact]

I think one of the most common structural materials in SR is plastiboard, which I'd imagine is about as thick as plywood, but more resilient since it's designed to flex rather than crack or shatter.  Proper demo there would likely involve removal and recycle rather than hammers and such.  The barrier table lists it as almost twice as hard to bust as plaster or sheetrock and likens it to ballistic glass.