For whatever it's worth, 8 U.S. presidents were lifetime members of the NRA. They were Ulysses S. Grant, Theodore Roosevelt, William Howard Taft, Dwight D. Eisenhower, John F Kennedy, Richard M. Nixon, Ronald Reagan, and George H.W. Bush. Kennedy stands out as the only Democrat on that list.
But I think @Herk has a point. A pro-2A president is nice and symbolically important, but I believe having majorities of pro-2A Congressional members is much more important legislatively. Presidents have executive orders that can be dumped by the next person in office and vetoes of legislation, which can be overridden.