Something I think that is as important as the weapon and caliber to carry which should be what your most accurate with .. is the placement of the security staff .. next to any entrance and several in the middle of the seating to be able to shoot outwardly also an AR in a elevated position with red dot might also be a viable option ..

also a written statement telling the congregation what they should do if a shooting incident does happen .. and what exit they should use if sitting in certain locations within the room .. you don't want people running into a security member causing his shot to be less then accurate and the results being an innocent person being shot ..