Here's some advice:
Obvious: get as many people to that meeting as possible...
Not so obvious: be very PASSIONATE...but, also very polite. Have a little get-together with the pro-freedom/gun people a few minutes before the meeting to explain this to them.
Anti-gun politicians are basically bullies. Also, they are always looking for reasons to make Second Amendment supporters look like wackos. The way the mood swings during the meeting will, most likely, determine how these coward politicians will vote.
Also, it helps to pay them a compliment or two during the meeting. "Thank you for having this meeting to discuss this important issue" or something like that. This allows them to save face and go with the mood of the meeting (which you've created) and lessens the resistance that might have been there if you hadn't given them a few compliments.
Good luck. Get enough people out to the meeting and appear to be the reasonable side, and you'll probably be alright.