The NRA is an extremely politically pragmatic organization and it is working very, very well for them. It irritates me to no end when they endorse and support incumbent Democrats against solid conservatives, but from the NRA's point of view if said incumbent supports their causes they will forgive just about anything else. This strategy has been successful. Where it once was a central Democratic plank, gun control is nearly dead almost everywhere. Rural and even suburban Democrats often won't even think of crossing the NRA, and those Democrats are rewarded with endorsements.
The NRA believes if it supports Willard, Willard will help advance their causes (or at least be better than Hussein). That is why they will endorse and spend millions upon millions of dollars to help get him elected. Regardless of what Mitt's done in the past, that is the calculation the NRA is making for the present.