I know a lot of people IRL with CCW permits that carry their sidearms in church. Both police and citizens alike. There have been a number of shootings in or at churches, and these police and CCW holders carry to protect their falimies them selves and other church members. I even know 2 pastors who carry as well. So it is not unrealistic and happens every day.
Though a sawed off shotgun IRL is extremely short range and inaccurate past a few feet. I prefer my Glock 30
ETA: The debate about guns and church comes down to what people believe about the 10 commandments. Some versions translate it "thou shalt not kill" which is a misstrans;ation of the original hebrew. Other versions translate it as "thou shalt not MURDER" and by murder premeditated slaying of another person. Not applicable to killing in war or in self defense. This is where the pacifist christians clash with the non-pacifist. If you want to do further reading on the subject here is a page with links to some articles:
http://keepandbeararms.com/information/XcInfoBase.asp?CatID=81
So in a non horror real life world there are good reasons to carry a firearm in church, and even MORE SO in a horror type world. Christ said: "But now, he who has a money bag, let him take it, and likewise a sack; and he who has no sword, let him sell his garment and buy one" (Luke 22: 36.)