Gregory,
Any killing of another person--whether through self-defense, war, or malice, is a sin, because it affects our relationship with God.
As Saint Basil pointed out, though, the consensus of the father is that killing in war is not murder. It's still wrong though (a necessary evil) and as such the person should forgoe communion for a time in order to have a chance to rectify the relationship with God. And of course there is a hierarchy of guilt: self defense, then wartime killing, then involuntary murder as a result of say driving drunk, and then a crime of passion, and then a premeditated act. Each level requires more penance.
You asked if Basil's canon was "in the canons." Yes, because Basil's canons are part of "the canons." However, in Basil's time penances were much more stringent. Nowadays the most I have ever heard of a priest giving a penitent to abstain from communion is 1 year--and this was from a monkpriest on Athos.
You also asked about artillery, etc. This is a very difficult question to ask, and one not really envisioned in the canons. My personal opinion is if you are in the battle helping the war you are guilty of the killing. Again, because you are just "following orders" you are not murdering and this is not an impediment to ordination. Still, the reality is that despite the fact that subjectively this is a "war situation" you are still taking a life (just like if someone broke into my house and I killed them, it would not be murder but it would still cause harm to my relationship with God and I would need to repent).
I'd just like to make it clear that I support armed forces and I love our military; what I am saying comes out of a basic understanding that sometimes in life we have to do something that is objectively wrong (i.e. killing) for the greater good. Of course, the only entity competant to order us to do this is the state; if we tried out vigilante justice it would be murder. Thus, participation in the armed forces remains an honorable thing for an Orthodox Christian to do.
anastasios