Bpt…
I think you would be very surprised to find out how many unassuming, quiet members of shuls (and parents of yeshiva kids) are licensed gun owners. Most of these gun owners go with some regularity to a range to maintain and improve their skills. It still always (pleasantly) surprises me, when I go, to see so many yarmulka wearing (many bearded) Yidden at the range with their own very serious firearms.
I think many, if not most of them would be happy to take turns in a rotation, to do guard duty at a shul and/or yeshiva. Since the police can NOT be everywhere, it would benefit the police (and the Jewish community) to provide additional training (after an additional background check) for these volunteers. This would be similar to the Mishmar Ezrachi in Israel, where ARMED civilians, AFTER formal training, patrol their neighborhoods, to supplement the work of the police and army.
Will it happen? Probably not. -At least not until, chalila, a tragedy occurs, – and THEN, it probably WOULD happen.