My first thought would be that anybody who thought the NRA was only about hunting rifles is tremendously ignorant of all the NRA has accomplished. To give just a few examples off the top of my head of what the NRA has done for the black rifle crowd:
The NRA was the first group to challenge 922(o) (1986 machinegun ban) in Farmer v. Higgins (11th Cir. 1990). They lost.
The NRA fought the original assault weapon ban in Congress from the time it was first introduced in 1989 until they were finally overwhelmed in 1994; however the NRA did succeed in inserting the 10 year sunset provision and introduced a repeal of the AWB that passed the House in 1996; but died in the Senate.
The NRA pulled support for their number one priority legislation in 2004 (Lawsuit protection for firearms manufacturers) rather than risk renewing the AWB after the Senate successfully amended the legislation with a ban renewal.
NRA Service Rifle Competition anyone?
Perhaps most importantly, without the NRA efforts in firearms safety and range construction, we wouldn't have both the tremendous base of knowledge on safe firearms use and a place where they can be safely used and enjoyed, regardless of whether that firearm is a single-shot hunting rifle or a semi-automatic detachable magazine rifle. Even worse, we wouldn't have a place to "grow" new shooters and without new shooters (i.e. votes) we would soon be marginalized. So you see, one of the greatest things the NRA does, doesn't even directly deal with firearms legislation.
Would you join and support an organization that worked to drastically reduce firearms legislation as it's one and only goal.
I'll support any organization that can show me results. The NRA, TSRA, and SAF are just a few of those organizations that offer a good return on investment in my opinion.