I'm not keen on weapons with options. I always end up worrying that I've made the wrong choice. Case in point: the grenade launcher. Should I fire frag or krak grenades? One gets me more goes with a worse grenade, the other gets me a single shot that might actually do something.
Fortunately, this is one of those things that we can have a fair crack at with the math-hammer. And here it is.
Frag vs Krak, chance to cause a wounding hit
Frag vs Krak, giving Krak a flat 2 damage to approximate a d3
Each chart shows a comparison of toughness and save (toughness down the side, save along the top). Where they meet, the number in the box represents the number of frags you'd need to fire to achieve the same chance of causing damage as a krak grenade. If the box says 2, for example, then 2 frags will achieve the same as a single krak. For the grid on the left, if the box is green, they'll do slightly better; if it's yellow, they do exactly the same.
But the right-hand grid is the more useful one. Here, a green box suggests that frag is the better option; yellow, that frag and krak are balanced; red suggests going with krak; and grey indicates that even 6 frags will be worse than a krak (statistically speaking). My thinking here goes like this:
If you only need two frags to match the krak, go frag. At three frags, we have a 2/3 chance of doing better with frag, so go frag. Four frags is the critical point. If four frags will do slightly better than a krak, it's a toss up - half the time, frag will be worse; half the time it'll be better. So you might as well go with your personal preference. But if four frags exactly match a krak, then go krak - now you have a 50% chance of doing worse with frag (rolling 1-3 shots), and only a 33% chance of doing better (rolling 5-6 shots). And once you need five or six frags, stick with the krak.
Curiously, looking at the upper grid (which is only concerned about causing damage of any kind), frag comes out ahead in almost every case. Krak is only the better choice against a 2+ save (if the target is toughness 4 or higher), and against toughness 6 (if the save is 4+ or better) - toughness 6 is krak's comparative sweet-spot, where it's still wounding on 4s but frag has dropped to 6s. So in most cases, if we're just looking to cause single wounds, frag is the way to go.
However, when we switch to the lower grid, there's a significant shift. On this grid, I've doubled the damage output of the krak grenade to represent its better damage characteristic - I've averaged it at 2 (rather than d3) to make life easier, but it does mean that this is something of an abstraction. In some cases (such as single-wound targets), this extra damage is wasted, but sometimes it matters (primaris are apparently quite popular). Broadly, this doubles the number of frags required (with rounding accounting for the appearance of odd numbers here). Suddenly, the weighting shifts heavily in favour of the krak grenade. Now, frag is only the better choice against toughness 1 (unless the save is 2+), and toughness 2 (if the save is 6+ or worse), and that's a ridiculously limited choice of targets - in practical terms, if we want to maximise damage, we should always choose krak.
Short version: if you're going after one wound models and want to spread damage around, use frag; if you want to inflict multiple damage on targets, go krak.
I know that sounds obvious, but the bit that surprised me was how universally that applies, especially with frag, In my head, frag is for low toughness, lightly-armoured targets. But the numbers suggest that, very limited scenarios aside, frag is the better option for single wounds, even against toughness 7 or higher. Or to look at it another way - the factor that primarily determines the grenade you should use isn't toughness, or save - it's the number of wounds on the target.
[There are more moving parts in this one, so I'm very open to argument.]
[And I have no idea how to make those opening images any bigger. Sorry.]
Edited by WarriorFish, 07 July 2020 - 08:50 PM.
Larger images added