They just actually endorsed him:
NRA Endorses Donald Trump at National Convention
The National Rifle Association endorsed Donald Trump at its national convention here on Friday, a late show of support from the nation’s biggest pro-gun-rights group for the presumptive GOP nominee.
But the endorsement was as much a show of opposition to likely Democratic nominee Hillary Clinton as it was a show of support for Trump. Before he announced the endorsement, NRA-ILA Executive Director Chris Cox told the crowd that “we have to unite and we have to unite right now.”