Railgun vs Ion Cannon

"The railgun is dead... and we have killed him" (Friedrich Nietzsche).

Okay, I may have modified the phrase slightly (and we all know Nietzsche played Chaos).  But an overused quote from a dead philosopher is always an acceptable opening to an article.

He has absolutely nothing to do
with this article, but my god look
at that magnificent mustache

The quote's dubious origin doesn't make it less true, though - railguns in the Tau arsenal are much less potent, despite the Hammerhead's version being unchanged in the new codex.

Hammerheads and their traditional railgun armament have always been a staple of the Tau army in past editions for a few reasons.  Their high armor value (for a skimmer) makes them very durable, and the railgun's is capable of bringing down heavy vehicles with its S10 AP1 shot or cutting through infantry with the S6 AP4 submunition's large blast.

None of this has changed in the 6th edition Tau codex.  While the loss of multi-trackers and the new disruption pod rules are detrimental to the Hammerhead, the drop in points cost more than makes up for these losses.  Being an inexpensive, durable, long-range vehicle with flexible firing options makes the vehicle a very attractive choice that fits into pretty much any Tau composition.

I'd like to stress here that I am not arguing railgun-armed Hammerheads are weak.  Everything listed above is absolutely true.  However, the railgun is eclipsed in every important way by the ion cannon because of the role a Hammerhead fills in a Tau army, and we'll go into that argument now.

Let's take a detailed look at how a railgun compares to an ion cannon:

Railguns offer 1 shot at S10 AP1, or a large blast at S6AP4.  Ion cannons offer 3 shots at S7 AP3, or a large blast at S8 AP3, with a chance of overheat.

The two important questions to ask here are 1) What does a railgun kill better, and 2) What should we be shooting with our Hammerhead?  Obviously the choice of weapon modifies question two, but bear with me for a moment.

More functional, though not nearly as stylish

S10 AP1 makes the railgun better at killing any AV 13 or 14 vehicle, granted; at AV 12, the multiple shots from the ion cannon more or less balances it out - the railgun is still better, but the difference is small enough to ignore.  The AP1 of a railgun also makes it more effective against anything with a 2+ armor save.

The ion cannon is better at killing everything else in the game.

That isn't a fair statement to leave on its own, granted - you do need something in your army to deal with heavy targets, and the railgun provides it.  But this is where our second question comes in - of those targets the railgun is more effective at killing, should you be firing the Hammerhead at them?  A single S10 AP1 shot is not a reliable way of bringing down a landraider, since it's reasonable to assume that a competent opponent will have given it some kind of cover or invulnerable save.  Even if the shot hits and gets through, you need a 5+ to penetrate, and a 4+ to destroy.  So, if we assume a 5+ cover save, you have a 2/3 * 2/3 * 1/3 * 1/2 = 2/27 chance of killing a land raider per railgun shot.

Goodnight, sweet prince

Things are even worse against models with 2+ armor.  Small targets like terminators are a terrible waste - you would most likely do more damage with a submunition than a solid shot against a unit with 2+ saves, so we'll leave them out of any calculations.  This leaves us with monstrous creatures, very few of whom have 2+ saves -  non-winged Hive Tyrants and Riptides being the notable exceptions.  The tyrant is just going to take the hits on his Tyrant Guard or shrug it off with FnP, making the railgun not particularly effective.  Riptides are unfortunately so durable (or not, you are playing Tau) that railguns are an enormous waste here as well.  Even if we assume the Riptide doesn't have FnP or is generating a 3++ save, a railgun shot has a 2/3 * 5/6 *2/3 = 10/27 chance of inflicting a wound.  You'd need three hammerheads firing for five turns uninterrupted to bring down just one Riptide.

Simply put, railguns are an enormously ineffective way of dealing with heavy targets because despite the weapons power, it's just one shot.  If you want to bring down heavy vehicles, you should be using fusion blasters, riptides in combat, or even EMP grenades - all of these are readily available to a Tau army, and are much more cost-effective ways of taking down vehicles.  Against monstrous creatures you'll find massed S5 firepower, plasma, fusion, or ion-accelerators are enormously cost-effective solutions than a Hammerhead.

To be fair, the numbers I list above could be easily modified via markerlights, boosting BS and negating cover to give the railgun a more reasonable damage output.  But consider that those markerlights could be spent just as easily on one of the alternative units listed above, and to much greater effect - why spend three markerlight points on a single Hammerhead when you could do the same for four Piranha?

Terran has been waiting 15 years
for this rules update

Compare this to the ion cannon.  Granted, 3 shots at S7 AP3 isn't overwhelming, but it's a solid choice against monstrous creatures, transports, and any single model with a 3+ save.  Most of the time, though, you're using the ion cannon for that beautiful S8 AP3 template.  Unlike the railgun submition, an overcharged ion shot is a threat against anything with a 3+ save and causes Instant Death against T4 models, making it especially effective against multi-wound units and infantry with FnP.

So in summary, the ion cannon is superior to the railgun because it lets your Hammerhead's do its job more effectively.  Most of the Hammerhead's power comes from its template, where ion has a clear advantage.  And while the railgun may be more useful against certain targets, it's important to understand that these are not the targets a Hammerhead is good at killing in the first place.

Leave other units to deal with the land raiders and riptides, and let your ion cannon do what it does best - killing everything else.