.410 Gauge

The .410 bore or .410 gauge, is the second-smallest caliber of shotgun shell commonly available. A .410 bore shotgun loaded with shot shells is well suited for small game hunting and pest control. The .410 started life off in the UK as a garden gun along with the.360 and the No.3, No.2 and No.1 bore rimfires.Wikipedia
 
Bullet diameter.410 in (10.4 mm) (slug)
Rim diameter.535 in (13.6 mm)
Neck diameter.455 in (11.6 mm) (plastic)
Base diameter.470 in (11.9 mm)
Overall length2″, 2 1/2″, 3″
Maximum pressure)18,130 psi
Place of originEngland