Sebastian at Shall Not Be Questioned has a blog post up analyzing the finances of the gun prohibitionists. The Form 990s have been posted and he has dug into them. They definitely had an influx of cash post-Newtown.
However, he ends with this:
We are doing a lot better than the media would credit us with but we can still do better. This is a fight where we can’t slack off because our opponents and their media lapdogs certainly won’t.