"I don't believe it's the NRA's problem to solve mass shootings."
Then what the hell are they good for?
When I was a kid (that was back when the NRA was still an advocate for the sportsman and before they became a shill for gun manufacturers) one of the NRA's primary tenets was to promote and facilitate gun safety, and I think we can all agree that mass shootings are about as unsafe as you can get with a gun. 
If the NRA was really concerned about gun safety, wouldn't you think that they would advocate AGAINST guns that would enable one person to kill and/or seriously injure more than 500 people in less than 10 or 15 minutes? Say YES.
Obviously, the NRA is not concerned about gun safety nor about you or me nor about the sanctity of the 2nd Amendment. All they care about is $$$$$$!

Best,

RTD