It’s not just the pro-gun issue, it’s his support of his party that is against gun ownership. One man does not make a party, but as part of a group he works against ownership of guns and the 2nd Amendment.
If they NRA always endorsed only Pubs, they would just be another Republican group. They would lose all their clout, and a lot of members. Their power comes from the fact that politicans know they have influence on independents and “bitter clinger” Democrats who care about gun rights, who are the very folks that decide elections.
My guess is they hedge their bets, and won’t endorse anyone.