/Ben brings in Tilted Weapons, Tilted Knowledge into one post:
There are different gauges of shotgun, and I have shot 12 and 20 gauge. I saw a 10 gauge once, but it looked too scary to fire.
The higher the gauge number, the smaller the barrel;
Why? The gauge is a measurement of diameter of lead balls that weigh one pound. These balls were often used in commerce and trade when scales were used but before a big standard unit of measure. Thus, 7 gauge means that 7 lead balls of equal size will weigh one pound.
12 gauge is popular for its ballistic properties, I guess.
Secondly, there are DOZENS of different types of "Birdshot", ranging from (help me out here, duckhunters) #3 to #9. The larger the number, the finer the pellets. #9 birdshot is like sand, I think.
The different types of birdshot produce different effects on different types of fowl at different ranges.
Therefore, if you are a hunter and you know exactly what you are doing, you would chose a 20 gauge over/under shotgun with #7 on top and #3 on the bottom.
Thirdly, there are different types of CHOKES available for your barrel. A choke is a little tube-like thing that concentrates (or disperses, whichever way you look at it) the pattern of pellets at different ranges. The Choke (and your barrel) are threaded, so you can change one for the other.
Types of chokes include Full, Modified, Sporting, et cetera.
Finally, to conclude: damage when hit at 30 yards with a shotgun blast (full or partial) would depend on:
A: Gauge of shotgun
B: Ammunition being used.
C: Choke pattern used in the barrel.
12 gauge, #3 birdshot with full choke would seriously fuck you up at 30 yards.
20 gauge, #9 shot with modified choke would not even make it that far. (I think, help me out here, birdhunters).