TIP 1.
Shotgun sizes are given in gauges, like 10g, 12g, etc. Gauge is measured the following way: You take a number of lead balls, the same width as the barrel, and add one ball at a time to a weight scale until it equals one pound (1lb).
For example, a 12 gauge shotgun would take 12 lead balls the same diameter as the barrel to weigh one pound. A 20 gauge would take 20 lead balls the same diameter as the bore to weigh one pound. And so on with other gauges.
What about the .410 shotgun?
The .410 is actually a .41 calibre, it is measured like a rifle bore in thousands of an inch.
TIP 2.
The problem which confronts the beginner in shotgun shooting is that of lead.
The recommended technique is that of bringing your gun up in the usual and then swinging it along the path of the moving target. You bring the muzzle from behind, tracking the target and catching up to it. As the muzzle passes it, you shoot.
It is most important not to slow or stop your swing. The swing and pull must be co-ordinated so that the swing will continue until the charge has left the muzzle. This automatically solves the problem of lead.
