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I think that would be interesting to do.
1 - everyone show up at a certain time and get a ticket immediately.
2 - gather on the sidewalk without entering
3 Once everyone is there with a ticket, start to go in, point out the sign
4 call for the manager, demand refunds for all
5 If you don't get the refund, give your tickets away to the people standing in line, along with a "No guns= no money card', or something like "AMC management will not allow me to provide for the safety of my family in their theatre, so I give this ticket to you with my compliments"
5a Possible the manager will remove the signs, at least for the night. If so, there might be some reasoning take place.

Pay for your ticket, then amble over to the door, read the sign carefully, then go back, ask for the manager, demand a refund.

"All 200 of us and our families are part of the local CHL fraternity, and we met here to go to the movie together. Would you happen to know where the next closest non-AMC move theatre might be?"