Here's the dope from Wikipedia, the on-line encyclopedia.

Your 18-1/2 inch barrel can accept all shot shells from BBB through No. 9 birdshot and No. 4 through 000 buckshot without any problem.

 

See the following write-up that helps explain gauge, birdshot, buckshot, and uses.  The No. 4 buckshot is considered the smallest size for combat use.  The most popular for police/military use is 00 buckshot, although the largest is 000, it has fewer pellets than 00. 

 

The intent was to allow you to size your buckshot by your tactical situation.  In close combat and building clearing, the No. 4 is preferred because it doesn't tend to over penetrate and is still lethal.  The 00 buckshot may over penetrate, but is considered optimum by police and military units for general purpose use.

 

Read this over and let me know what questions come about as a result.

Bob

 

Shotshell sizes

Shotgun shells are generally measured by "gauge." While "bigger" caliber designations incrementally increase the width of a rifle barrel, gauge refers to the division of a pound of lead into equal sized spheres: the fewer number of spheres the pound is divided into, the greater the diameter each will be.

For example, a shotgun is called 12 gauge because twelve lead spheres, each of which just fits the inside diameter of the barrel, weigh one pound. Each gauge is the count of the number of lead spheres that weigh a pound and which just fit into the barrel. This measurement comes from the time when early cannon were designated in a similar manner—a "12 pounder" would be a cannon that fired a 12 pound cannonball.

No.of lead balls in one pound	diameter of lead balls
10	0.76" (19.7 mm)
12	0.73" (18.5 mm)
16	0.66" (16.8 mm)
20	0.62" (15.6 mm)
28	0.55" (14.0 mm)

The most popular shotgun gauge by far is 12 gauge, with other common gauges being 10, 16, 20, and 28. There are also some shotguns measured by diameter, rather than gauge, these are the .410 (10 mm), 9 mm (.357), and .22 (5.5 mm); these are correctly called ".410 bore", not ".410 gauge". Shotshells are also found in some handgun cartridges, such as .38 Special and .44 Magnum; these are often used in revolvers for defense against snakes at very close ranges, or for killing small pests such as rats. A number of single shot pistols and rifles are made in .45 Colt with special screw in chokes allowing the use of .410 bore shells—usually the chokes are designed with deep grooves parallel to the bore designed to stop the spin of the shot column, as the .45 Colt barrel is rifled.

Shotgun gauge	bore diameter
10	0.76" (19.7 mm)
12	0.73" (18.5 mm)
16	0.66" (16.8 mm)
20	0.62" (15.6 mm)
28	0.55" (14.0 mm)

Shot sizes

Shotshells are loaded with different sizes of shot depending on the target. For skeet shooting, a small shot such as a # 8 or #9 would be used, because range is short and a high density pattern is desirable. Trap shooting requires longer shots, and so a larger shot, up to #7½ would be desired. For hunting game, the range and the penetration needed to assure a clean kill must both be considered. Shot loses its velocity very quickly due to its low sectional density (see external ballistics). Small shot, like that used for skeet and trap, will have lost all appreciable energy by 100 yards or meters, which is why trap and skeet ranges can be located near inhabited areas with no risk of injury to those outside the range.

Birdshot

Birdshot sizes are numbered similar to the shotgun gauges; the smaller the number, the larger the shot. Generally birdshot is just called "shot", such as "number 9 shot" or "BB shot". A useful mnemonic for remembering the diameter of numbered birdshot is simply to subtract the shot size from 17. The resulting answer is the diameter of the shot in hundredths of an inch. For example, number 2 shot gives 17-2 = 15, meaning that the diameter of number 2 shot is 15/100 or 0.15". B shot is .170 inches, and sizes go up in .01 increments for BB and BBB. While B and larger are often considered buckshot, they are also used on birds at long ranges.

Size	Nominal diameter	Pellets/oz
		Lead	Steel
BBB	.190" (4.83 mm)	44	62
BB	.180" (4.57 mm)	47	72
B	.170" (4.32 mm)	50
1	.160" (4.06 mm)		103
2	.150" (3.81 mm)	87	125
3	.140" (3.56 mm)		158
4	.130" (3.30 mm)	135	192
5	.120" (3.05 mm)	170	243
6	.110" (2.79 mm)	225	315
7½	.095" (2.41 mm)	350
8	.090" (2.29 mm)	410
9	.080" (2.03 mm)	585

Birdshot selection

For hunting, shot size must be chosen not only for the range, but also for the game. The shot must reach the target with enough energy to penetrate to a depth sufficient to kill the game. Lead shot is still the best performer for the money, but with environmental restrictions on the use of lead, especially with waterfowl, steel, bismuth, and tungsten composites are required. Steel, being significantly less dense than lead, requires larger shot sizes, but is a good choice when cost is a consideration. Steel, however, cannot safely be used in some older shotguns without causing damage to either the bore or to the choke of the shotgun due to the hardness of steel shot. Tungsten shot is equal or even greater in density than lead, but is far more expensive. Bismuth shot falls in between steel and tungsten shot in both density and cost.

Game	Lead/Tungsten	Steel
Turkey, pheasant	4 to 6	5 to 6
Quail, dove	7½ to 8
Rabbit	6 to 7½
Squirrel	6
Geese	BB to 2	BBB to 1
Ducks, low	4 to 6	2 to 4
Ducks, high	2 to 4	BB to 2

Buckshot

Larger sizes of shot, large enough that they must be carefully packed into the shell rather than simply dumped or poured in, are called "buckshot." Buckshot is used for hunting larger game, such as deer (hence derivation of the name), and also in riot shotguns and combat shotguns for defensive, police, and military use. Buckshot is also categorized by number, with smaller numbers being larger shot. It is called either "buckshot" or just "buck", such as "triple-ought buck" or "number 4 buck".

Buckshot may be illegal in certain jurisdictions. For example, buckshot of a greater diameter than four millimeters is illegal for hunting purposes in Germany.

Size	Nominal diameter	Pellets/oz
000 ("triple-ought")	.36" (9.1 mm)	6
00 ("double-ought")	.33" (8.4 mm)	8
0 ("one-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