Sheriff's (and other non-federal officers) don't enforce federal gun laws (and don't have the authority to do so), unless they are adopted into state/local laws. Sheriff's that came out against the federal gun laws were simply making a name for themselves to get votes.
This is different then the Sheriff's who came out against the State of Colorado gun laws, those laws are within their authority, big difference.
It would be like if the Seattle City Council came out with a law that said driving a red car is illegal in the city limits, and having the County Sheriff say he won't enforce it. Well the only people that can enforce municipal/city code are city PD Officers, so the Sheriff doesn't even have the authority to enforce it in the first place.
