Hello again! This is Russell back again to talk about what I think is an under appreciated model in the new Tyranid codex: the Tyrannofex. Please note, however, that I don't claim that it is a "good" model, just one that I run frequently, and that I think more people should run. Now that I've lost all credibility by admitting to running a model that isn't good, let's talk about the choices for unique weapons that the Tyrannofex can carry.
His name is Jake. Say "Hi Jake!"
I think a lot of people are taken in by this very sub-par weapon. Assault 20 does sound rather intimidating, I'll admit. But let's look at it this way: what else has Assault 20? Twenty Gaunts. And those gaunts are less than half the cost, can hold objectives, could take a Mycetic Spore, and are debateably harder to kill due to their ability to get cover saves and the fact that they have 20 wounds (kind of). Honestly, I think 20 Gaunts is probably a better choice than the Fleshborer Hive Tyrannofex, and I don't think 20 Gaunts is a particularly good choice.
I don't have a lot of experience with this particular weapon choice, though it seems reasonable. It makes your Tyrannofex very good at pushing stubborn units off of an in-cover objective. Eldar Rangers or Imperial Guardsmen with Camo Cloaks can be frustrating for Tyranids to get off objectives, and Acid Spray, along with the Thorax Swarm the Tyrannofex comes with, can be very effective at killing them. But this does make your Tyrannofex into a very short range, very slow beast who will spend the first several turns just moving up the board doing nothing. I think that Acid Spray is worth considering, if you are having trouble with your opponents going to ground in cover on objectives, but I am not sure that it justifies the very high cost of the Tyrannofex.
This is the thing that really sells the Tyrannofex for me. Two shots at BS 3 is nothing impressive, and I do warn you that this weapon will miss at frustrating times. But a long range, high-strength weapon is something that the Tyranids are completely lacking outside of the Tyrannofex. When you really need a tank across the board to die, and can't get the Hiveguard or Zoanthropes there in time, the Tyrannofex is a great backup plan. I have had great success breaking open transport tanks with the Tyrannofex so that my Genestealers can assault the embarked troops. The Rupture Cannon is also a good answer to some high speed, long range weaponry, such as the Imperial Guard Valkyrie/Vendetta, which can outrun and outrange your other tankbreaking options fairly easily. All in all, I think that the Rupture Cannon is the best option for the Tyrannofex.
The Tyrannofex is a very expensive model, and I would not recommend trying to squeeze it into any list below 1500 points, and even then it is questionable. But it can be very effective as a long range gunner. I also usually move it slowly up the board, using its Cluster Spines and Thorax Swarm in the late turns once the tanks are all down. I have also found that the Tyrannofex rarely dies, because the amount of firepower that the opponent has to pump into it to bring it down is usually not worth the amount of damage that it does.
All in all, the Tyrannofex is not actually a good model. It is very expensive and does not do very much damage. But I feel justified in taking it on a regular basis simply because it fills a hole that the Tyranids are not able to fill with any other model. Tyranids are sorely lacking in long range, high-strength shooters, and the Tyrannofex is an overpriced option, but it does the job, and honestly that is good enough to justify its points in a lot of games.