You might, for a second, look beyond defending our 2nd Amendment rights and ask what else the different groups do.
The NRA wasn't created to defend our rights. It wasn't something needed in 1871. And wasn't needed for nearly 100 years afterwards. Sadly, it's been badly needed since, and we have not won every fight.
The NRA promotes and provides safety and shooting training. That was their mission, and still is. Becoming politically active wasn't the intent, but it has become the necessity.
NRA certified safety training is recognized by the states, and required in some cases (like getting your first hunting license, etc.)
If, for some reason, you are only going to support one group, consider the group that does more than just fight the political fight.