I'd dispute that assumption. Plenty of liberal/progressive type(s) own guns, are enthusiasts, train regularly, and own multiple firearms. From my perspective; If conservatives stood alone in the value(s) of the second amendment, there would have been an amendment banning/restricting guns already in place.
Conservatives alone cannot win this fight. So, no matter whether Johnny Reb or INGO feel about the issue, you're in bed with many liberals on this one. As an aside, I will posit that I believe more conservative are single-issue voters than liberals.
The comment was MOST gun owners are conservative. Not all. That said, given the true representation of liberals in the gun owners community, I'll just say every gun group has to have at least one token liberal.