Wiktionary
a. 1 (context of a firearm English) Having a small caliber. 2 (context by extension English) insignificant or trivial.
Wikipedia
Small-bore refers to calibers with a diameter of .32 inches or smaller. It may also refer to small-bore rifle competition. The medium-bore refers to calibers with a diameter between .33 inches up to .39 inches and large-bore refers to calibers with a diameter of .40 inches or larger. Miniature bore historically referred to calibers under .22. There is much variance in the use of the term small-bore which over the years has changed considerably with anything under .577 caliber considered small-bore prior to the mid-19th century.
Usage examples of "small-bore".
If you have to fire or feel you have to at a suspicious something, then please do it with a swivel gun, a small-bored one, or better yet, with a caliver loaded with ball and shot.
The broadside turrets were echeloned to allow at least five infinite repeaters to engage on any target bearing, and while the ion-bolt armament was far less powerful than the small-bore Hellbores mounted as secondary weapons aboard current model Bolos, Maneka knew the 10-centimeter version mounted in the Mark XXVIII would penetrate better than a quarter-meter of duralloy at close ranges.