Vladimir Kushnir describes how Symantec's censorware blocks access to many pro-gun websites such as nra.org, under the category 'Weapons'. I checked, and he has accurately described which sites are blocked. (Though the Lott site that is blocked is actually a collection of Lott's articles---Lott's own site is not blocked.) I agree with Kushnir that most people would not expect 'Weapons' (blocked by default) would block access to sites like gunscholar.com.
Kushnir also found that many more pro-gun sites were blocked than pro-control sites. I found that 13 of 48 sites in Google Directory's Pro-Gun Rights category were blocked by the 'Weapons' category, while only 5 of the 78 sites in Google's so called Anti-Gun Rights category were blocked. There certainly seems to be some bias against pro-gun sites here. Kushnir believes that this
could only be achieved as a result of a conscious effort to block conservative websites, dedicated to the protection of the Constitution, while not blocking the liberal ones.
I don't think this is correct. They do block some pro-control sites and the pattern observed could be the result of unconscious bias on the part of the employees selecting sites, or that pro-gun sites are more likely to be submitted for blocking.
In any event, I don't think any of the sites should be blocked (and perhaps the NRA and VPC can agree on something here---they are both blocked). But then, I think the whole idea of censorware is a bad idea.
Update: Guncite (one of the blocked sites) has more information. Symantec's explanation for the blocking was:
Basically the logic behind gun filtering lists came about after the Columbine school shootings. It was decided that I-Gear would start filtering gun sites that promote gun use so schools can monitor their students in hopes of preventing future school shootings. This is the reasoning behind filtering sites that promote gun "use" vs. guns in general.