Had I not been automatically voting for all Republicans in every general election no matter what, I would have decided who to vote for in 2012 this way:
a) is the Democratic candidate pro-gun (or even moderately pro-gun)?
If yes, vote for the Democrat. If no, go to B.
b) Which candidate is the most pro-gun?
Vote for that candidate.
In other words, I would have supported Obama if he was somewhat reasonable on guns, even against a Republican candidate who is even better on guns.
I also would have supported Obama if the Republican candidate was worse than him on guns.
But I would have applied the same calculations had Obama been the challenger and Romney the incumbent. It would have had nothing to do with the election being a referendum on the incumbent. I just used to try to support pro-gun Democrats wherever I found them, in an attempt to swing the Democratic Party away from their hatred of civil liberties.
I still try to support pro-gun Democrats in primaries even today.