Birdshot sizes are similar to how shotgun gauges are measured, but they use different terminology. The one thing they have in common though is that the larger the number, the smaller the shot; and vice versa. When you refer to a birdshot you would say something like a “number 2 shot...
Buckshot can damage a full choke tube or shotgun barrel because the pellets can not compress tight enough to travel through the choke. This may result in the choke or barrel bulging as it gives way to the shot column passing through. ...