ORIGINAL: driftrider
The first number is the gauge of the shell. Gauge refers to the number of pure lead balls at standard bore diameter required to make one pound. This is why a 20 gauge is smaller than a 12 gauge. Obviously, one must use shells of the same gauge as their shotgun.
Of course, one number refers to the length of the shell. 12 gauge shells generally come in 2 3/4", 3" and 3 1/2". This is important because some shotguns cannot safely chamber shells longer than a certain length. On modern shotguns, the chamber length will almost always be printed on the barrel. NEVER use a shell longer than the stated chamber length.
DRAM EQUIVALENT
The accepted method of correlating relative velocities of shotshells loaded with smokeless propellant to shotshells loaded with black powder. The reference black powder load chosen was a 3 dram charge of black powder, with 1 1/8 oz. of shot and a velocity of 1200 fps. Therefore, a 3 dram equivalent load using smokeless powder would be with 1 1/8 oz. of shot having a velocity of 1200 fps. or 1 1/4 oz. of shot and a velocity of 1165 fps. A 3 1/4 dram equivalent load might have 1 1/8 oz. of shot and a velocity of 1255 fps. Abbreviated Dram Equiv.
Shot sizes are a little trickier. Bird shot will be numbered from 9 shot and down, with the larger number indicating a smaller pellet diameter. Once you get below #1 birdshot you get into the lettered shot sizes like "T", "B", "BB", etc... These are larger shot sizes generally used only in non-toxic waterfowl loads, though I have seen "BB" lead shot loads too. Then there is the buckshot. These are much larger pellets, with even more confusion, with sizes like "0" (ought), "00" (double ought), #4 buckshot (not anywhere close to #4 birdshot), etc...
Birdshot
Size, Nominal diameter, Pellets per oz (28 g)Lead, Steel, Quantity per lb.
FF, .23" (5.84 mm),--, 35, --
F, .22" (5.59 mm),--, 39, --
TT, .21" (5.33 mm),
T, .20" (5.08 mm),--, 53, --
BBB, .190" (4.83 mm), 44, 62, 550
BB, .180" (4.57 mm), 50, 72, 650
B, .170" (4.32 mm), --, --, --
1, .160" (4.06 mm) , 72, 103, 925
2, .150" (3.81 mm), 87, 125, 1120
3, .140" (3.56 mm) , 108, 158, 1370
4, .130" (3.30 mm), 135, 192, 1720
5, .120" (3.05 mm) , 170, 243, 2180
6, .110" (2.79 mm), 225, 315, 2850
7½, .095" (2.41 mm), 350, --, 3775
8, .090" (2.29 mm) , 410, 686, 5150
8½, .085" (2.15 mm) , 497, --, --
9, .080" (2.03 mm), 585, 892, 7400
Buckshot:
Size, Nominal diameter, Pellets/oz
000 ("triple-ought"), .36" (9.1 mm), 6
00 ("double-ought"), .33" (8.4 mm), 8
0 ("ought"), .32" (8.1 mm), 9
1, .30" (7.6 mm), 10
2, .27" (6.9 mm), 15
3, .25" (6.4 mm), 18
4, .24" (6 mm), 21
Hope this helps,
Mike