Mark M
9mm is the most common caliber for blank, semi-auto guns. The blank ammo is commonly available in regular and extra-loud. 9mm may also fire .380 blanks, sometimes called 9mm half loads, which would be quieter. In general, the smaller the caliber the less powder capacity and quieter the load, so .22 would be the smallest/quietest, provided you can find a semi-auto style gun in the caliber. You might want to look into engaging a firearms consultant/instructor with theatrical experience, even if limited to a phone or email engagement. They will probably know sources for different stage guns and blank loads, and can offer other advice. I'm sure your insurance carrier, actors and crew would also appreciate the expert advice. Even blank guns can cause burns and eye injuries if mishandled.

Bear Crap
You can go with a .22 blank gun here is a store with all kinds; Or you can fake it with background sound effects; Or you could use these; Just pull the string and it makes a bang sound like a small firecracker. Just have a couple people set them off as the person points the “gun”. A 9mm would be a bit loud I would use the .22.

Jennifer
Check out gas powered airsoft or CO2 BB guns. Nice little pop, but not too loud for indoors. And you can get CO2 BB guns at walmart fairly cheap

HajiRaj
You may want to look into a realistic cap guns. Seriously.Quieter and less chance for any type of accident. Good luck.