Rory, I would suspect they are incorporating statements the candidates have made regarding firearms. It would be nice if they included links so in cases like this you could have a better understanding of their reasoning and make an informed decision about the accurracy of the rating they assigned.
The following ratings are based on lifetime voting records on gun issues and the results of a questionaire sent to all Congressional candidates; the NRA assigned a letter grade (with A+ being the highest and F being the lowest).
Source: NRA website 02n-NRA on Dec 31, 2003
Bernie Sanders on Gun Control
Click here for 7 full quotes on Gun Control OR background on Gun Control.
Voted YES on banning high-capacity magazines of over 10 bullets. (Apr 2013)
Voted YES on allowing firearms in checked baggage on Amtrak trains. (Apr 2009)
Voted YES on prohibiting foreign & UN aid that restricts US gun ownership. (Sep 2007)
Voted YES on prohibiting product misuse lawsuits on gun manufacturers. (Oct 2005)
Voted YES on prohibiting suing gunmakers & sellers for gun misuse. (Apr 2003)
Voted NO on decreasing gun waiting period from 3 days to 1. (Jun 1999)
Rated F by the NRA, indicating a pro-gun control voting record. (Dec 2003)
http://www.ontheissues.org/Senate/Bernie_Sanders.htm