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According to the Ork FAQ, Kannons are now ordinance weapons when using frag rounds. This has a few problems.
First, the codex specifically states that frag rounds use the small template, but the rules for ordinance weapons overrule that, so the Kannon now uses a large blast template. This seems overpowered to me, so I suspect we're still supposed to use the smaller template.
A more complex problem arises when we consider a GunTrukk with a Kannon and Big Shoota. When firing frag rounds, the kannon is firing an ordinance weapon and thus can't use the Big Shoota. So for all intensive purposes, we can use the Shoota just fine against light vehicles, but can't use it against massed infantry without emasculating the Kannon. That just doesn't make sense.
I'm fairly certain that the GunTrukk wasn't considered when they released the FAQ, and that GunTrukks should still be able to fire both.
What are people's opinions on this?
Well Kannons are ordanence. Although you can't use the big shoota on the gun trukk it's no big deal your firering a large template so it evens out real good. You don't really need to fire the big shoota anyway. It hardly ever kills a thing so who cares.
I think that, in the end, what was decided with this issue was "we're going to pretend, as ork players, that the thing uses the smaller blast template until our codex says otherwise".
I stand by this because lack of 'uge sodding template weapons is one of the weaknesses inherit to the ork list (well, at least until you loot something...). Also, it's my understanding that any other weapon with a similiar profile as the frag round uses the small template (mortors, for example).
"Some days you're the Mekboy building the kannon, the rest you're the grot being blown up by it."
- Quote attributed to Sorkrates, before his much mourned death due to ingestion of Hemsquig Juice.
"Dis iz my choppa, and herez me gun. Dat's for killin' and so'z dis one!"
- Bloodaxe Kommando Warchant
Nothing anywhere says than an ordinance weapon uses a large blast template. It was the case in 3rd ed but not now. Ordinance as a weapon descriptor simply dictates how the act of shooting is handled, what table to use in the event of a penetrating hit, and if other weapons may be fired by the vehicle carrying it. There is another descriptor that tells whether or not the weapon uses a blast template and if so what size.
Look at the statline for the Kannon and then compare that to the statline for the Demolisher cannon in the new marine codex. They are both Ordinance and they both tell you which size template to use. Pretty simple, eh?
Artificial Intelligence is no match for Natural Stupidity.