In regards to odds to hit a tank...
Then with a small blast, you have a "small" radius, plus the "radius" of your target, so let's say 3.0" altogether, on the narrow side of a Chimera / Rhino. Technically, you might do a little better, as certain placements can allow a larger scatter to still hit, as small portion of the time. So you then have a 66.7% x distance less BS on 2d6.
Assuming you have a BS 3 Model, a distance of "6" or less should generate a hit. [Large blast would be a 7 ]
0.333 + [0.667 x 15/36] = 61.1% chance to hit, with a small blast.
Str 9 vs 12 has a 2/3 chance to strip a hull point so that's 40.73 hull points stripped. Huh. Interesting. That's terrible compared to the devourers, but there's no chance for a pen and no chance to damage AV 13+
Hm. I can see why people run the duel devourers.
Man, now I want to run 2 sets of 2 carnifexes. One brood full assault, and the other full devourers.
Bioplasma still isn't bad though, even just playing head games. But if I run that, I'm trying to think if there's anything useful I can do with the extra pair of arms. Deathspitter is cheap and has a good range.... huh. Hmm.... yeah, duel devourers. I see why people run them.
But against AV 13+... the only thing you can do it hope to assault them. Hmm. I'm arguing with myself at this point.
..... Now I want a tyranofex with a rupture cannon. I'm so sad that I'm poor....
Here's a list that I'm pretty happy with with my current collection. I'm just wondering where to go from here. I think its still weak at long range tank hunting. But maybe that's something that Nids can't do?
Fyrant - Wings, Toxin Sacs, Bonesword, Whip (Hold in reserve and then comes in and assaults)
Tyrant - Heavy Venom Cannon, TL Devourers (Warlord. Tanks the middle and sits behind Aegis next to venomthrope for a 2+ cover save!)
Tyrant Guard x2
Genestealer Brood x5 + broodlord (infiltrates and horrors people and then joins an assault)
Termagants x15 - Devourer x5 (Screens for tyrant and hangs out midfield)
Hormagaunts x17 (Runs up with CC Fexes)
Warriors x3 - Deathspitters x2, Venom Cannon (Mans quadgun and holds home objective)
Warriors x3 - Devourers x2, Barbed Strangler (Filler synapse and skulks around and grabs other objectives)
Venomthrope x1 (Hides behind tyrant and does his thing)
Sporeminds x6 (Deep strikes and makes the opponent go "oh crap")
Carnifex - 2 TL Devourers
Carnifex Brood x2 - Spinebanks, Bio-plasma, Adernal Glands
Aegis Line with Quadgun
That's a lot of wounds....