Stupid people want Mark Kelly to take away their GUN RIGHTS!
Gun owners are a diverse group. Unfortunately, too many are notorious for sitting on their backsides until after an election. When they finally wake up and realize they're facing yet another ban, they spend $$$$$$$ on over-priced hardware that's about to become illegal, instead of sending $ or $$ to a pro-gun candidate like McSally and preventing the ban in the first place.
Gun owners can be the most powerful political force in the country. After the 1994 Republican wave, voting statistics in USA Today (IIRC) suggested that between 1/4 and 1/3 of ALL voters had been gun owners voting Republican. (The Cinton gun ban had been a MAJOR wake-up call!)
It's just a matter of waking them all up, and that takes effort; you have to address the issues. In my experience, it never, ever happens with candidates who try to 'triangulate' and attract 'moderate'/undecided voters by pushing "Democrat-lite" policy positions. Offering a choice between Democrat & "Democrat-lite" simply validates liberal arguments, and too many voters just pick Democrat..