Frangible .22 Shorts or CB caps if shooting indoors.
Only time I've fired a .22 indoors it was to do prelim sighting in after altering a rear sight and having sanded away a lump in the barrel channel that had caused wild shots. I used CB caps.
It worked well enough that the sights required no further adjustment to get in the ten ring at 50 yards, further adjustment to get in the X was quick and easy.
Bore sighting would have done almost as well. But I was snowed in and bored.
PS
Using a .22 short to kill a rat can end up costing hundreds of dollars in plumbing repairs and water damage, don't ask how I know this.
