I'd wait until shots were fired or until it was painfully obvious (to everyone) that someone was about to be injured or killed. I would make sure that I was in a situation where a no-bill from the grand jury was a cake-walk. With my luck, if I acted sooner, under the ski mask it would be some 15 yr old kid with a mail order replica gun.
Lots of folks know I'm "into" guns. I would be a big target for a DA wanting to make a case against someone who was "just waiting for a chance to shoot someone." I shudder to think what the (totally legal) contents of my gun safe would look like spread out in a courtroom.