If I have Autocannon and Shadow Self already in play, and I play a creature from my hand so that it next to Shadow Self, what happens? Does the creature eat the damage, does Shadow Self absorb the damage, or do I get to decide?

I am guessing that autocannon's "enters play" takes place before the creature "lands" next to Shadow Self so that Shadow Self can't take the damage, but I'm not sure, and I can't find it in the FAQ.

As it says on Autocannon, the 1 damage that it deals to a creature is dealt after it enters play.

As the creature is in play and next to Shadow Self, the damage is dealt to Shadow Self.

I agree that's one interpretation, but while the rules contain a glossary entry for "Leaves Play", they do not have an entry for "Enters Play". It seems possible to me that "Enters Play" effects go before "Play" effects, in which case one could argue that "Enters Play" must happen before the card hits the table. I'm not trying to say that's what I think the rule is or should be, just that it's not as clear to me as it seems to be to you.

"After it enters play" is at the same time as Play: effects; they just use that terminology when it talks about something else entering play. And in accordance with the FAQ, the Shadow Self won't even take that damage if the neighboring creature has armor when it enters.

2 hours ago, andrew144 said:

I agree that's one interpretation, but while the rules contain a glossary entry for "Leaves Play", they do not have an entry for "Enters Play". It seems possible to me that "Enters Play" effects go before "Play" effects, in which case one could argue that "Enters Play" must happen before the card hits the table. I'm not trying to say that's what I think the rule is or should be, just that it's not as clear to me as it seems to be to you.

I mean it doesn’t matter if “enters play” has a 12-step process, the auto cannon still does damage after it finishes.

Shadow Self is very upset that you would deny it the very purpose it was created for.

1 hour ago, Rabbitball said:

"After it enters play" is at the same time as Play: effects; they just use that terminology when it talks about something else entering play. And in accordance with the FAQ, the Shadow Self won't even take that damage if the neighboring creature has armor when it enters.

Good catch, that FAQ ruling convinces me.

While I'm glad this got figured out, I must confess I find myself flabbergasted that armor preventing damage was the catalyst. What did I miss here?

Play creature from hand -> creature enter the battleline -> adjacency effect triggers -> ETB effect triggers (autocannon) -> Play effect of creature triggers.