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Monday, April 18, 2011

Storm Bolter Statistics

I keep running into this nugget all over the place, and basically I wanted to make sure folks have a little perspective on this:

What's the difference between a normal storm bolter and one with psy-bolts added?

Storm Bolter: % chance to kill a space marine:
0.666 (to hit) * 0.5 (to wound) * 0.333 (chance to fail save) = 0.111

Storm Bolter w/ Psybolts: % chance to kill a space marine
0.666 (to hit) * 0.666 ( to wound) * 0.333 (chance to fail save) = 0.15

Difference: 3.9%

So, as you can already tell, neither load out is exactly frightening. Granted, attaching psy bolts increases the effective killing power versus marines of a storm bolter by about 35%, let's put that in perspective.

Any given Storm Bolter ( 2 shots ) has a 19.2% chance to kill at least one Space Marine.

Any given Storm Bolter w/ Psybolts ( 2 shots ) has a 27.8% chance to kill at least one Space Marine.

So, here's some perspective. How many "less" Storm Bolters will psy-storm bolters require to reliably kill the same amount of normal Storm Bolters?

Well, we need to define "reliable". Let's say that reliable = 75% chance of success. This means that 3 out of every 4 times you try it, it works.

So, here's the rephrase. How many Storm Bolters and Psy-Storm Bolters will each respectively take to reliably (75% chance) to kill at least one marine?

To figure this out, you calculate your odds of killing nothing and subtract that from one. (Dirty statistics will tell you it takes 9 shots). The actual answer is approximately 13 shots to reliably kill one Space Marine, that's 6 and a half Grey Knights firing just to reliably take down one dude. (Have some perspective here as to why I'm not a huge fan of the supposed Grey Knight Pew Pew tactic from Storm Bolters.)

For a Psy-Storm Bolter? The answer is 9 shots. So, that's 4 and a half Grey Knights firing to reliably take down one dude.

My overall point? Clearly, neither of these load outs is all that effective at killing marines.

P.S. Don't even bother shooting at something with FnP. :)


  1. As you rightly pointed out, I don't think the purpose is for MEQ kiling.

    Just a suggestion, but have you looked at the increase in anti vehicle firepower?
    The fact is, the basic GK squad could now Pen armor 10 (looking at you, DE, orks, etc) and in fact, 3 GK give you a 50% chance to get at least one pen, and 6 gk give you a 75% chance to get at least one pen. and you can now glance rhinos, speeders, and other armor 11 at the same rate.

    I doubt it would kill much besides some DE vehicles, but it really adds some ability to shake and stun vehicles, preventing a lot of return firepower against your low model count army from a normally meched up field.

    Perhaps an analysis of their effect anti vehicle, and the fact that they can supress alot of the return fire they would normally see, is needed? I'm not saying i know it's worth it, but i think that the math might be interesting, and it could be a possible very effective use....

  2. Certainly the ability to glance an Armor 11 vehicle is pretty cool as is added ability to penetrate an AV 10 vehicle. It's not an optimal use of the weapons, but will work just fine in a pretty low quantity. It takes about 4 GKs firing (if the vehicle is not in cover) to have > 50% chance to get a 6 on the armor pen roll. This means it will only happen 1 in 2 times. If you want it to happen 3/4, you will need about 6 marines firing. Not bad.

    That being said, the psycannon is what will truly weight that fight and of course be the more reliable solution. A psycannon only shooting twice has a 56% chance to glance/pen an AV 11 vehicle. So, one guy with a psycannon as far as AV 10/11 lockdown even on the move = about 4-5 marine with psy-bolter storm bolters.

  3. I guess my problem with this post and similar statements is theres no frame of reference. eg what constitutes a good MEq killing percentage and what weapons have that at a reasonable cost, availability, etc?

    A 4 in 27 chance to kill a marine sounds poor, but thats for a single shot and not 2 shots and it is assault 24" (meaning to me it has more opportunities to be used compared to rapid fire or 12-18 ranged weapons). Compare it to other options.
    --15 in 27 chance for BS4 plasma gun at 24". But the model has to stay still and if MEq, 1 in 18 chance of dying to overheat.
    --And that 15/27 gets cut in half if the target is in cover. Guess what -- that puts it almost on par with a psySB. Infact at 24" with a target in cover you'd need just about as many plasma guns as psybolters to kill a single marine 75% of the time. Lot easier to get 4 psybolter GKs then it is to get 4 PGs in another SM army.
    --Idea above carries over to a host of other S6+ AP3 or better shooting.
    --7 in 27 chance to kill a marine per assault cannon/psy cannon round. Excellent considering you'll have anywhere from 2-16 shots from typical units (not to mention TL on some vehicles). However again cover reduces that to about 5.5 in 27 chance. Still better but not so much that you wouldn't want an extra bit of chance via the psybolters.

    Other considerations:

    --Volume of fire that basic weaponry puts out as a whole can make up for the lack of full on killing potential. You play orks and mention this idea alot when it comes to their cc ability. Similar thing applies here more shots so more potential for beyond the bell curve results and more reliable average results.

    --A SSGK 10 man psybolt and 2 psycannon squad standing still. The psybolts contribute more to the avg MEq kills of the unit then the psycannons. And the unit is more points efficient then without them (more kills per point).

    Certainly other factors come into play (rapid fire for plasma, instakilling, 2+ saves, FnP, can't use mass SBs while embarked, etc) so I am not saying psybolters are almost as good as plasma or anything like that. I guess mostly all I am saying is for MEq killing bolters seem pointless with a 1/9 (or even 4/27) until you see that plasma isn't auto-slaying marines either and on top of that begin taking volume of fire and cover into account.


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