BB Caps would be the most quiet. CB caps are probably easier to find on store shelves. The Aguila .22 Kolibri makes very little sound, I have regulated scopes on .22 rifles in my back yard using those, not much more noise than a pellet gun. I have recently seen .22 CB at Wal-Mart and .22 Aguila Kolibri at Academy. Academy seems to stock numerous Aguila-made calibers.