What does fullbore mean?

'Fullbore' basically means a larger calibre, or bullet size. "Fullbore" is usually taken to mean calibres over .30" - but of course some calibres are measured in mm rather in inches!

At our shooting club we usually shoot .308" (known as '308'),  and 7.62mm (known as '762')

Smallbore target rifle is usually .22 calibre which is often shot indoors but can be shot outdoors as well.

Another difference is that most smallbore cartidges are 'rimfire' (the firing pin strikes the outside edge of the cartridge)  whilst fullbore is always 'centre fire' - the primer is located in the center of the cartridge case head and is usually replaceable so the cartridge can be reloaded and used again.