When using chaingun from the Punisher I can target a group of individuals that are adjacent to each other and keep shooting even if I miss a character. Meaning I miss a character but I can just move on to the next adjacent character. So the only way the chaingun stops is when the target characters are KOed or you roll a critical miss. Is this correct?

CHAINGUN: Once per turn when the Punisher succeeds at a ranged combat attack against an opposing target character, after the attack resolves he can immediately make ranged combat attacks against any other opposing characters adjacent to the target to which he can draw a clear line of fire. (Each attack is rolled and deals damage separately). If the result of a successful ranged combat attack roll made by the Punisher is doubles, the damage dealt is penetrating damage.

It says "Once per turn"
so you can only use the power once per turn.

The power activates when he succeeds attacking the first target.
Once he hits the first target he can attack other figures adjacent to the first one that he has LoF to.

If he misses the first target then its no go on the power.

That is what I thought as well but another player at our venue stated that he read on the realms that even if you miss you still attack. I need to find this ruling.

it does not exist.

You need to find a ruling that completely overturns the text on the card? (I am assuming that the text Maraud quoted is correct.) Any such "ruling" would be contained in Errata or FAQ documents. Any such "ruling" made "on the realms" would be useless unless backed up by official documentation.

For what it's worth, I concur with Maraud's interpretation.