"Once per round, after you perform an attack that does not hit, you may perform an attack with an equipped (cannon) secondary weapon."

my question is:

does this mean if you only have, say, a laser cannon equipped and your first attack that misses was with this cannon, do you get to fire this cannon again? or does it need to be a different cannon?

"Once per round, after you perform an attack that does not hit, you may perform an attack with an equipped (cannon) secondary weapon."

my question is:

does this mean if you only have, say, a laser cannon equipped and your first attack that misses was with this cannon, do you get to fire this cannon again? or does it need to be a different cannon?

The card doesn't specify which weapon you need to use for the first attack - only the second.

Provided you have a cannon upgrade equipped and can legally take the shot you're fine.

so for example if your only available shot was a range 1 shot and you only had a heavy laser cannon equipped then you would not get the second shot.

Note that the ability does not restrict you to shooting at the same target though

I think his question is can he use the same cannon for the first and second shot.

do you get to fire this cannon again? or does it need to be a different cannon?

Yes, it doesn't say anything about a different cannon, only that you must use a cannon.

Do cannons fall under the once per turn rule.

Do cannons fall under the once per turn rule.

No. That rule only applies to Actions, and Bombs, and nothing else I can think of.

It's not the "once per turn" rule that covers it, it's "once per opportunity", and IG-88B's ability offers a new opportunity if he misses with the first shot.

Do cannons fall under the once per turn rule.

There is no general "once per turn" rule. The only thing that could qualify is that you can only use an action once per turn, but clearly that doesn't apply.

He's got it confused with the "same action once per round rule". Easy mistake, but still doesn't apply to this.