Small-bore refers to calibers with a diameter of .32 in (8.1 mm) or smaller.
[1] It may also refer to small-bore rifle competition.
[2] Miniature bore refers historically to calibers of .22 in (5.6 mm) or smaller.
There is much variance in the use of the term small-bore, which over the years has changed considerably with anything under .577 in (14.7 mm) caliber considered small-bore prior to the mid-19th century.
Small-bore competition often refers to shooting competitions conducted with .22 Rimfire target rifles.