GURPS Explosives: Custom Shrapnel and other ways to deal more damage

[Icelander]

Originally Posted by MrFix

I suppose, I just need to figure out the weight and how much explosive/shrapnel material I have to acquire for a game where I'd play a demo expert. I doubt a GM would permit me a 6dx8 satchel charge with [7d] fragments just for the price of explosive and weight of explosives (11.4 lbs)

No, but even if you are getting very inefficient fragmentation because you are making this in the field, you should be able to match a mature TL6 (using TNT and steel, you are effectively making a TL6 quality weapon, even if your skills are higher TL) with maybe twice the weight of fragmentation material.

So, 6dx6 [6d] cr ex is very plausible at around 50-lb of explosive and fragmentation materials. For a character with expert level (15+) skills in Engineer (Bombs), Explosives (Demolitions) and Machinist, as well as access to the right workshop, you could match this in a 25-lb bomb.

Even with just Explosives (Demolitions) and a stop at a hardware store to buy 10+ lbs. of nails, screws or the like, you should easily be able to add [4d+2] fragments to your bomb.

[MrFix]

And I suppose using C4 instead of TNT shaves off 3.5 pounds of weight or so, leaving me with 21.5 lbs bomb.

What if it's something much more simple, such as nails, buckshot pellets, what have you, something much easier to procure and attach to the plastique? I am trying to play a demo expert, rather than outright engineer.

[Icelander]

Originally Posted by MrFix

And I suppose using C4 instead of TNT shaves off 3.5 pounds of weight or so, leaving me with 21.5 lbs bomb.

What if it's something much more simple, such as nails, buckshot pellets, what have you, something much easier to procure and attach to the plastique? I am trying to play a demo expert, rather than outright engineer.

It's just less efficient in terms of weight and bulk, but ultimately, you can do similar damage.

Just make sure that it's at least 20% heavier than a purpose-built, sleek-and-efficient bomb and probably 50-100% heavier if you're just using drop in components.

It's efficiency that is expensive in terms of effort, tools and skill, but anyone with Explosives (Demolitions) knows how to use nails, screws, ball bearings or the like to add fragmentation damage. Basically, just make sure to use enough.

[MrFix]

On topic of nails etc, I suppose 6.5 lbs of C4+10lbs of filler is enough to create a 16.5 lbs 6dx6 [4d+2] satchel charge?

[Icelander]

Originally Posted by MrFix

On topic of nails etc, I suppose 6.5 lbs of C4+10lbs of filler is enough to create a 16.5 lbs 6dx6 [4d+2] satchel charge?

Well, it sounds pretty reasonable to me.

I haven't plugged the C4 into the explosive damage equation and reduced it for some of the explosive force being directed at scattering fragments, but it looks broadly reasonable by comparison with other examples.

Perhaps whether you need a couple of pounds more explosive and/or five or ten more pounds of metallic anti-personnel fragments might be determined by the results of Explosives (Demolition) skill checks, depending on your GM's judgment, but this is certainly within the realm of possibility.

[MrFix]

Alright, thanks. I suppose limiting shrapnel by bomb's "weight caliber" is the solution. I'll use that.

I do welcome others to contribute if you have a different idea of how things should go, if there's some kinda way to extrapolate a formula from data or some such.

[DouglasCole]

Just as an upper end, and very unique comparison: the 2,700-lb shell of the 16" guns of the Iowa-class battleships are only 41 lbs of explosive.

The purpose of those is "putting holes in armored ships," of course.

The "High Capacity" shell is "only" 1,900 lbs total, 155lbs bursting charge.

That's one designed to hit bunkers and make big holes in land things.

So depending on the mission, the casing/fragmentation-to-charge ratio can be pretty skewed.

[Kromm]

When considering fragment lethality, don't overlook that most fragmentation attacks generate multiple hits. You're not looking at trying to get a fragment to kill somebody, but at trying to get two or three or more fragments to kill somebody. For instance, a single 2d(0.5) pi- pellet from an M18A1 isn't going to kill most targets . . . but that's a highly directional device, and at the mentioned 5 yards, the average victim is going to eat seven pellets, which is rather another matter.

As a broad rule for dedicated high-explosive munitions (not just solid ones with bursting charges), at a given TL, explosive damage goes up with the square root of explosive weight, while fragmentation damage goes up roughly with the cube root of (device weight - explosive weight). You can optimize for one or the other by altering the proportions. This is where skill comes in.

So you could get 1-lb. devices like these:

• 1 lb. TNT in a cardboard casing of negligible weight: 6d×2 cr ex
• 0.75 lb. TNT in an 0.25-lb. shell designed for light fragments: 5d×2[1d] or 5d×2[1d+1] cr ex
• 0.5 lb. TNT in a 0.5-lb. shell designed for moderate fragmentation: 8d+2[1d+2] cr ex
• 0.25 lb. TNT in a 0.75-lb. heavy fragmentation sleeve: 6d[2d] cr ex

The above assumes omnidirectionality. Directional devices are better modeled on shot shells.

[Icelander]

Originally Posted by Kromm

As a broad rule for dedicated high-explosive munitions (not just solid ones with bursting charges), at a given TL, explosive damage goes up with the square root of explosive weight, while fragmentation damage goes up roughly with the cube root of (device weight - explosive weight). You can optimize for one or the other by altering the proportions. This is where skill comes in.

So you could get 1-lb. devices like these:

• 1 lb. TNT in a cardboard casing of negligible weight: 6d×2 cr ex
• 0.75 lb. TNT in an 0.25-lb. shell designed for light fragments: 5d×2[1d] or 5d×2[1d+1] cr ex
• 0.5 lb. TNT in a 0.5-lb. shell designed for moderate fragmentation: 8d+2[1d+2] cr ex
• 0.25 lb. TNT in a 0.75-lb. heavy fragmentation sleeve: 6d[2d] cr ex

In published examples, adding a fragmentation sleeve or improvised filler often reduces pipe-bomb, grenade, booby-trap or IED damage by around 20%, up to 33% in one case.

Got any guidelines for how to apply that effect to PC-made satchel charges with Home Depot fragmentation materiel?

[Eukie]

Originally Posted by Icelander

Explosives don't kill people. Fragments kill people.

Blast overpressure can definitely kill you. Fragments are often better at killing, but explosives will kill too.