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besides when only one can draw LOS to a decent target, of course.

I'm talking against armor here, as grouping infantry under templates depends on actual game situations.

So, when is it better to shoot two S9 AP2 shots vs a single S10 AP1? 

*edited for single S9 shot percentages*

If the vehicle can be penned

AV 10

S9: 2/3 hits * (1/6 glance (with 1/6 kill) + 5/6 penetrate (with 1/2 kill)) = 2/3 hits * 1/36 + 2/3 hits * 5/12 = 0.5/27 + 7.5/27 = 8/27 (roughly 29.6% per shot)

S10: 8/9 hits * 1/1 chance to pen (with 1/2 kill) = 8/9 * 1/2 = 4/9 (roughly 44.44%)

AV 11

S9: 2/3 hits * 1/36 + 2/3 hits * 4/12 = 0.5/27 + 6/27 = 6.5/27 (roughly 24% per shot)

S10: 8/9 hits * 1/2 (still auto-pens) = 4/9 (roughly 44.44%)

AV 12

S9: 0.5/27 + 4.5/27 = 5/27 (roughly 18.5% per shot)

S10: 8/9 * 5/12 = 40/108 (roughly 37%)

AV 13

S9: 0.5/27 + 3/27 = 3.5/27 (roughly 13% per shot)

S10: 8/9 * 4/12 = 8/27 (roughly 29.6%)

AV 14

S9: 0.5/27 + 1.5/27 = 2/27 (roughly 7.4% per shot)

S10: 8/9 * 3/12 = 6/27 (roughly 22.22%)

VS skimmers moving fast

AV 10

S9: 2/3 * (1 * 1/3) = 2/9 (22.22% per shot)

S10: 8/9 * 1/3 = 8/27 (29.6%)

AV 11

S9: 2/3 * 5/18 = 5/27 (18.5% per shot)

S10: 8/9 * 1/3 = 8/27 (29.6%)

AV 12

S9: 2/3 * 4/18 = 4/27 (14.8% per shot)

S10: 8/9 * 5/18 = 20/81 (24.7%)

AV 13

S9: 2/3 * 3/18 = 3/27 (11.11% per shot)

S10: 8/9 * 4/18 = 16/81 (19.8%)

AV 14 (if such a thing exists... )

S9: 2/3 * 2/18 = 2/27 (7.4% per shot)

S10: 8/9 * 3/18 = 12/81 (14.8%)

The moral being... don't use the combined shot when firing at fast moving skimmers if you can draw LOS from both Prisms.

That's as far as I'm taking it tonight.  Interesting numbers...

VS AV12 Chimera:

% chance of doing nothing with 2 S9 shots: 30.86

% chance of doing nothing with 1 S10 shot: 25.93

In order to do "nothing", both shots would have to miss or fail to glance; the percentage is way lower than that.

Yeah what the hell? My calculator is broke. I'll redo it.

Edit: Are you sure it's wrong, I've redone it.

Lets use 1 S10 shot as an example.

Chance of missing = 1/9. Chance of hitting but not penetrating = 8/9 * 1/6.

1/9 + 8/9 * 1/6 = My answer above.

Edit 2: You said "both shots" meaning you think the other answer is wrong.

It isn't. I just spent 10 minutes doing every possible roll (there are 1296 rolls). 400 result in nothing. 400/1296 = My answer above.

Guess I figured wrong... Oh well. Sorry.

Sorry kwade but the calcs on two shots is wrong. Assuming you're looking for a percent chance of atleast 1 destroy result.

Example AV10:
1 shot chance to destroy 1 vehicle:
(2/3 hits * 1/6 glance * 1/6 destroy) + (2/3 hits * 5/6 pen * 1/2 destroy) = 8/27 chance to destroy

2 shot chance to destroy atleast 1 vehicle:
1- chance of not destroying with two prism shots  = 1-(19/27 * 19/27) = ~50.48%

Your off because your percentage is including the chance of destroying 2 vehicles (which is ~8% for AV10).

I also mention this to illustrate another reason to consider 2 shots can be better then a combined shot, because you can potentially destroy a vehicle with the first hit and proceed to blast something else with the second.

Heh, no need to appologize for correcting me Winter.

You're correct, I was a bit lazy by not taking into account the fact that the first shot could potentially get a destroyed result, meaning that the second shot could go somewhere else.

However, when you get over AV 13 I think the calculations are still useful.  Perhaps not spot on since there's the chance to get two 'killing blows' in there, but interesting none the less.

Perhaps the easiest thing to do would be to halve the S9 percentages, which gives you the chance per shot of getting a destroyed result.  Math holds up, and now you're looking at the raw data for getting multiple kills as well.

I definately agree that against anything AV12 and below, 2 shots are better than 1.  The always pen result from combining the shots, however, makes me lean toward using a combined shot for anything above that (and it gives a nice ranged option for trying to take down things like Monoliths - a traditional bane of Eldar).

Time to edit.