1. If in a situation where you might need the gun for defense, why is it even a shailoh?
2. Based on the mishna discussing someone who wanted to wear their dress uniform (parade armour and parade weapons – not the ones that are designed to be used in combat), it seems very clear that you can only carry weapons based on military necessity, not to show off. That suggests perhaps a humrah to conceal the weapon rather than to show off you have a gun. A terrorist in fact would first target anyone with a gun so there is no advantage to the armed person in displaying a firearm, and it is a shailoh if displaying a gun is in fact a way of impressing your neighbors, in which case the mishna’s objections to displaying weapons would come into play.