It's fairly clear that Buck is far better than the likes of Lindsey Graham, John McCain, Bob Corker, Thad Cochran, Lamar Alexander, etc on key issues like the economy, world government and civil liberties. He seems on par with Jim DeMint, whom many considered a liberty-minded candidate. I highly doubt the rest of the GOP Senators would have received 19/20 in the Campaign for Liberty survey.
Vocal hawk on Iran, yes. Remember, even Mike Lee is a hawk on Iran. More than half the people on this list are hawks on Iran. If Iran is a dealbreaker, then the only incumbents we should support are Amash, Rand, Massie, Jones, Sanford and Duncan (TN) and the only people who can be deemed liberty candidates are ideological acolytes of them. I don't consider that to be the case, most of those incumbents don't consider that to be the case and Ron Paul/CFL/YAL don't consider that to be the case either.
Buck has
"expressed disapproval over America's involvement in Iraq" and is
opposed to national building in Afghanistan.
The drug war is mostly a non-issue at the federal level at the moment, there's usually at most 1-2 votes on it a year. Buck is right on so much and it would be unfortunate to oppose someone who agrees with us on so much because of a few minor disagreements.