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First time Demo has come up in our game and I am still a bit green on how the rules/mechanics convert into a visual decription of what is wanted. So any feedback is welcome.

My players are in the sewers on their way to their target. Along comes a swarm of Devil Rats from a side tunnel. My Demo players says she wants to create a cave-in on that side tunnel using her rating 10 Foam explosives. We did not go into too much detail with the game formulas, barrier ratings, and such. With hindsight I think that was a bad idea, but did not want to slow down our pace. In the back of my mind was that the 2Kg she said she used would not be capable of caving in a tunnel. Am I wrong?
I'd say 2 kg is more than enough, unless the tunnel is made out of seriously reinforced concrete. But I'd worry about the backblast issue. Detonating a large stash of explosive in the narrow confines of a sewer tunnel falls straight into the 'very bad idea' category in my opinion.
Placed in boreholes it probably could. sprayed on a wall, probably not. Sprayed and tamped, maybe. Depends on what is behind the lining in all cases.
Depends... As i understand it the demolitionist has no time to prepare the charges, can set no holes and stuff. So technical he can't use the demolition skill to enhance the attack.

So let's say he can just spray it on somehow before the rats eat him:

He has 2 KG of R10 explosive: that makes it a 14+DV explosion. (root of KG times rating). Against structures it counts double (so 28DV). The walls is likely weak structural (brick) with an armor of about 12 (so it buys 4 hits and the DV goes down to 24).

24 Damage is enough to blow a chunk out of the walls which measures about 2 meters²*10cm.

Of course that is just pure rules... and we all know the rules are... a bit inaccurate. There are other things to think about: Confined space, so maybe more pressure (Chunky Salsa?), other distribution of damage (deeper but not so much spread?).

A good rule is: However you feel like it. I would allow it, because i like to have my games a bit like a cheap television-show, and this feels like a good setup to have all things collapse and have the PC's run for their lives *G*.
Well, let's make an assumption that the character being played is smarter than all of us on the subject of "how to blow stuff up properly" (unless a Dumpshock forumer is actually a demolitions expert, in which case I apologize). In my mind, I'm a bit wary of realistically trying to cause a cave-in that actually seals off an area, as I've watched too many episodes of Mythbusters, but let's just assume that the character really can do this (this is, after all, Shadowrun, and we are prone to playing with ridiculous situations). Given that there are no specific rules on how to cause a cave-in (explosives are generally used to make holes in things, not move matter from one place to another), we'd have to make something up as a GM.

The way I would handle this is "If you can make a hole in it, then you can cause a section of the sewer system to collapse equal to the volume of the hole being blasted." The problem is coming up with enough debris to block devil rats (which means not just a cave-in... it means sealing it off). How big is your sewer tunnel? Some sewer tunnels are two by two meters in the cross section. If the sewer tunnel is particularly narrow, you could probably get away with less, but I'd go for about 4 cubic meters of matter (2 by 2 by 1).

Now for the rules:
1) Determine the material you are trying to blow up. It will have two essential properties: Armor (which is rolled against the damage of the explosive, although SR4A and I both recommend using Armor divided by 4 to reduce DV, simply to make things easier, using the 4 dice = 1 hit rule) and Structure (how much damage is required to make a 1 meter by 1 meter hole). These values vary wildly by what structure you are trying to blow up. In your case, a crumbling old sewer or a new sewer built by the lowest bidder would probably be made of brick or plascrete, with values of 12 Armor and 11 Structure (p166 SR4A). A new sewer would probably be closer to reinforced concrete with values of 24 Armor and 15 Structure. Let's be nasty and say it's 24 Armor and 15 Structure (if it's lower, then all of the calculations below would actually work as well).
2) Determine the size of the blast needed. To do this, you'll have to figure out the volume needed to cave-in and seal the sewer. As above, I think 4 cubic meters of matter is appropriate (a 2 by 2 tunnel, with 1 meter of debris blocking the way). Unfortunately, Shadowrun demolitions rules gives everything as a radius, forcing us to think in spheres. Fortunately, a 1 meter sphere is 4/3 pi radius cubed, which gives us a bit over 4 cubic meters (4.19ish, I think). Since we are probably blowing out hemispheres of matter (only 2 cubic meters, half of the previous values), we would need two charges (that can blow out 1 meter radius) on separate locations (probably on each side of the tunnel or possibly in series on the ceiling).
3) Let's go to the table on p92 Arsenal! Conveniently enough, it has both Rating 10 Explosives AND Armored/Reinforced Material listed (reinforced concrete). It says you would need about 2 kg of Rating 10 explosives to blow a 1 meter radius hole**. For a pair of holes, you'd probably need 4 kg of Rating 10 explosives. D'oh! But this table assumes that you are using an untamped charge just sitting on the surface. If you tamp it, have it placed higher above the ground than the hole radius, in water (sewers! But we don't want to create a crater, we want material to cave into the tunnel!), or inside the structure, you need half the material or less. I'd assume a demolitions specialist would know this (at least, how to tamp a charge).

** EDIT: It's actually 1 meter diameter hole, which is MUCH different than a 1 meter radius hole... it just goes to show that the demo rules are very complicated! That, and I either suck at reading or at math. I'm inclined to say the former.

Unfortunately, this all took about 15 minutes of calculation, by which point your players have completely fallen asleep and long for the days in SR3 and SR2 when all they had to wait for was the decker to "do his Matrix thingy". The ad-hoc rules that I use in SR4 are: If the Rating of the Explosive equals the Structure Rating of the material, then a successful Demolitions roll with 1 kg of the Explosive will blow a hole through it (1 m x 1 m). If it is less than the Structure Rating, you will have to get a minimum threshold of the difference between the Rating and the Structure on the Demolitions test (which works because you get additional DV from the Demolitions test when placing explosives). I just assume that the Armor doesn't come into play if you have a properly placed charge (especially since you can reach an obscene x4 multiplier with the appropriate knowledge and positioning). This is rough and inaccurate compared to the existing rules, but it allows you to quickly calculate on the fly.

For further "rules in play", make your demo player learn all of the demolitions rules and memorize it by heart. You would expect a hacker/mage/technomancer/any other character to know the rules of their own specialty, and so you should expect the player to follow the same standard.
Look at the goal of the players and decide if it seems reasonable. The goal of the player was to stop the devil rats and her method was to seal off the tunnel. I think you would have been perfectly acceptable not allowing a cave in, but the blast either killed or frightened them away. At the end of the encounter the effects were different, but the outcome was the same. So it doesn't matter if the tunnel came down or there was a bunch of smoking debris and charred rats. The players were safe.

Just remember this is a game and not engineering so your decision to move the game along was a sound one. Maybe there was a loose roof, voids and poorly compacted material above the tunnel. Maybe when she did a blast it broke up the hoop causing differential stress on the wall allowing water pressure to bring it down.
Thanks much,

Summerstorm and Hahnsoo both made the rules in the book much more understandable for me. A bit too much math in Hahnsoo's case but it still made much more sense to me vs. reading over the books at 2am my time. I honestly forgot about the enclosed space idea, that would have made the beginning of the run more interesting. (We stopped playing after the remains of the swarm made it to the team.)
If you think SR's rules on blowing up walls are bad, take a peek at Alpha Omega's sometime. A stone wall (we were blowing up a charge that fell down a crevass in a cave) and the way "armor" works is two values:

How much damage you throw out (Damage Threshold)
How much damage the armor takes (Damage Resistance, which is damage dealt to the structural integrity of the armor)
Remainder hits your flesh

And then there's armor piercing, which reduces DT and DR.

So the explosive had an AP of -20, damage ~65 out to 15 meters (halved at 8 ) against the stone's values:

DT: 4 per inch
DR: 12 per inch
SI: 30 per inch

So AP -20 means that DT and DR are both 0, which means that you deal no damage to its SI (people behind 1" of stone take full damage and the stone is left intact)? Is damage of the blast reduced or not (stone vaporized provides no cover or does 1 kg of explosive only devastate ~2 inches of material)? Do the DT and DR values stack per inch before reduction from AP (AP ignores the DR/DT from the first inch and part of the second)?
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