CNN Money article: The Money Powering the NRA
From a small-town pharmacist to a commercial pilot, it's not just gunmakers that are funding the National Rifle Association’s political battles.
The organization's overall revenue, which includes membership dues, program fees and other contributions, has boomed in recent years – rising to nearly $350 million in 2013. The majority of this money funds NRA initiatives like member newsletters, sporting events and gun safety education and training programs.
These help the NRA recruit new members and spread its pro-gun message. But to influence laws and keep its chosen leaders in power, it has a separate pool of money to use.
Also follow this link from the bottom of the above article: The NRA's money: 10 things we knowSome political funding comes from big corporations, many within the gun industry, which donate millions to the NRA. But companies are barred from donating to the NRA’s political action committee, which the agency uses to fill campaign coffers, run ads and send out mailers for and against candidates.
What this article DOESN'T point out is that there is, for all practical purposes, only ONE person providing money for the gun control side. (AFAIK)
Of course, there is also the Joyce Foundation.

