Gun Owners of America.
Second Amendment Foundation.
While NRA has its problems, it is a 600lb gorilla - and it is the only 600lb gorilla in the fight. No one has as much lobbying power as the NRA and no one even comes close. So my advice is that every gun owner should belong to the NRA and at least one other organization that they believe in. In this way, each gun owner is represented currently and supports a smaller organization trying to come up in the fight and make a difference.