My goal of this post is to get some MathHammer on the title's subject, now that 6e has changed things a bit.

As I recall, with 5e eliminating 'partial' hits for templates (4e's game mechanic), the submunition round surpassed the ioncan for MEQ killing, given that power armored goons tended to clump up more during 5e as they did in 4e, getting out of rhinos & drop pods. My experience at least.

With 6e's game mechanic of Focus Fire assisting the IonCan's AP3, where does the MathHammer now fall? I'm not number savvy enough to plod through odds calculations. Drift for the scatter, and all that makes my brain hurt.

Could someone help out with this? How many spess muhreens die under the following circumstances when shot at by the Sub munition Round vs. the Ion Cannon? (if you're good enough at MathHammer to include MarkerLight fun, then da-ang, go for it!).

a. The SM are in the open and reasonably clumped together, so 6 would fit under the Big Blast Marker.
b. The SM have some Cover. 3 or 4 do not. Apply Focus Fire
c. All have Cover (5+)
d. All have Cover (4+)

So, 8 different situations.

Well really the Railhead has never been much of a MEQ weapon. Say you hit 6 with the blast, you wound 5, kill 1-2. Not that amazing. Where it comes in is as a horde killer and fire magnet. A Str6 AP4 cover ignoring large blast wipes out units of hordes but isn't so effective against anything that gets their armor save. The ion cannon has always been better at dealing with regular marines esp when it ignores cover. The ionhead @ BS4 hits 2 wounds 1-2 then kills 1-2 depending on cover. Same effect for less cost.

Keep in mind though, blasts can now be used to snipe the special weapons out of a team thanks to the new wound allocation removing models under the blast.

@Carrelio - Unfortunately, blasts only remove casualties from under the center of the blast first if it's a barrage weapon. Page 33 specifically notes that "any unsaved wounds are then allocated on the unit as for a normal shooting attack". It isn't until you get to the next page's notes on barrage weapons that it says "when determining wound allocation, always assume the shot is coming from the center of the blast marker". This *is* the case with the AFP, though, so it's still worth keeping in mind.

As far as the original question goes, a marine hit by a submunition blast has a (5/6 * 1/3) = 5/18 = ~27.8% chance to die. A marine hit by an ion cannon has a 5/6 =~83.3% chance to die, assuming no cover save. So:

With no cover (or focusing fire on marines out of cover), an ion cannon kills (2 * 2/3 * 5/6) = 10/9 = ~1.11 marines.
With 5+ cover, an ion cannon kills (2 * 2/3 * 5/6 * 2/3) = 20/27 = ~0.74 marines.
With 4+ cover, an ion cannon kills (2 * 2/3 * 5/6 * 1/2) = 5/9 = ~0.56 marines.

The submunition is much harder to quantify. If you assume you get 6 under the template on a hit, how many do you get on a scatter? When it scatters, you've got a 6/36 = 1/6 chance to not scatter after all (roll of 4 or less), a 15/36 chance to scatter 3" or less (roll of 7 or less) and a 15/36 chance to scatter 4" or more (roll of 8 or more). Obviously you still hit 6 when it doesn't scatter after all. As for the rest of it, my own personal guess might be that you'd get 2-3 marines on a small scatter and 0-1 on a large one, depending on the direction of scatter and arrangement of unit, but this is *very* much a guess.

So, with the caveat that this is *extremely* sketchy guesswork, under the assumption that you get 6 marines 4/9 of the time (1/3 chance not to scatter + (2/3 * 1/6) = 1/9 chance to roll a 4 or less), 2.5 marines 5/18 of the time and 0.5 marines the last 5/18 of the time, that's an average of 7/2 = 3.5 hits per shot. At a kill rate of 5/18 per hit, that's an average of 35/36 = ~0.97 marines killed per shot (cover being irrelevant for this weapon).

In other words, under this extremely simplified set of assumptions, the ion cannon outperforms the submunition against marines out of cover, but does worse against marines in cover, which doesn't exactly surprise me. You do raise a good point, though, that with relatively small shot count armor ignoring weapons like the ion cannon, the new ability to focus fire does significantly improve your chances of getting those no-cover shots. Ultimately, though, I don't think I'd be very likely to drop the railgun for an ion cannon unless I was *really* hurting for points. It may be easier to exploit the ion cannon to full potential against basic marines these days, but I don't think it's likely to outperform the submunition by enough to make up for the loss of versatility.