Shotgun Gauge vs. Diameter

The gauge or bore of a shotgun, as well as the bore of some large dangerous-game rifles, does not refer to the actual diameter but rather to the mass of a pure lead round ball of a diameter equal to the bore. For example, a 16 gauge round lead ball weighs 1/16 lb, or one ounce.

Here is a list of gauges with the corresponding bore diameter (caliber) and weight of a lead ball of that diameter. Source: Wikipedia

Gauge (Bore)	Caliber		Weight of unalloyed (pure) lead ball
	(mm)	(in)	(g)	(oz)	(gr)
AA*	101.60	4.000	6225.52	219.6	96080
A½*	76.20	3.000	2626.39	92.64	40530
0.25*	67.34	2.651	1814.36	64.000	28000
0.5*	53.45	2.103	907.18	32.000	14000
A*	50.80	2.000	778.19	27.45	12010
0.75*	46.70	1.838	604.80	21.336	9328
1*	42.42	1.669	453.59	16.000	7000
B½*	38.10	1.500	328.3	11.58	5066
1.5*	37.05	1.459	302.39	10.667	4667
2*	33.67	1.326	226.80	8.000	3500
3*	29.41	1.158	151.20	5.333	2333
4*	26.72	1.052	113.40	4.000	1750
B*	25.40	1.000	97.27	3.43	1501
5*	24.80	.976	90.72	3.200	1400
6*	23.35	.919	75.60	2.667	1166
6.278	23.00	.906	72.26	2.549	1114
7*	22.18	.873	64.80	2.286	1000
8	21.21	.835	56.70	2.000	875
9*	20.39	.803	50.40	1.778	778
10	19.69	.775	45.36	1.600	700
11*	19.05	.750	41.24	1.454	636
12	18.53	.729	37.80	1.333	583
13*	18.04	.710	34.89	1.231	538
14*	17.60	.693	32.40	1.143	500
16	16.83	.663	28.35	1.000	438
18*	16.19	.637	25.20	0.889	389
20	15.63	.615	22.68	0.800	350
22*	15.13	.596	20.62	0.728	319
24*	14.70	.579	18.90	0.667	292
28	13.97	.550	16.20	0.571	250
32*	13.36	.526	14.17	0.500	219
36	12.85	.506	12.59	0.444	194
C*	12.70	.500	12.16	0.429	188
40	12.40	.488	11.34	0.400	175
67.62	10.41	.410	6.71	0.237	104

The asterisk denotes a rarely-encountered bore size.