Complex sentence: The.410 bore is the smallest shotgun size which is widely available commercially, in the UK 9mm shotguns are quite common for use in "GardenGuns" and folding shotguns.
Split into simpler sentences: The.410 bore is the smallest shotgun size which is widely available commercially. In the UK, 9mm shotguns are quite common for use in "GardenGuns" and folding shotguns.