In looking, I can't find any proof though that the income mix has changed, as least using 2004 as the earliest base time.
About half of the NRA's funds come from dues - this seems consistent. The other half comes from a variety of incomes, most dependent on gun manufacturers (advertising in NRA magazine, donations from gun manufacturers, percentage of gun sales, etc).
What is interesting is that the NRA benefits from every mass shooting attack. After an attack, gun purchases go up (each contributing the bottom line directly), donations from gun manufactures go up to pay for lobby to stop any gun control and magazine enrollment spikes (helping directly and with advertising revenue).
The NRA also directly benefits anytime there is mention of gun control in terms of donations; to the point that the NRA actively misquotes positions to make it seem politicians want to "take away your guns".
So while it may seem odd to see Wayne La Pierre (the $1,000,000.00+ / year spokesman) gets more strident after each mass shooting - it makes financial sense.