Gauge
From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
Type	Shotgun
Place of origin	Various

The gauge (in American English or more commonly referred to as bore in British English) of a firearm is a unit of measurement used to express the inner diameter (bore diameter) of the barrel.

Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm and is expressed as the multiplicative inverse of the sphere's weight as a fraction of a pound, e.g., a one-twelfth pound lead ball fits a 12-gauge bore. Therefore with a 12-gauge, it would take 12 balls of lead of the same size as the 12 gauge shotgun's inner bore diameter to weigh 1 pound (454 grams).^[1] The term is related to the measurement of cannons, which were also measured by the weight of their iron round shot; an eight-pounder would fire an 8 lb (3.6 kg) ball.

Gauge is commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 2 bore during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, called "elephant guns", were intended for use primarily in Africa and Asia for hunting large dangerous game.

Gauge is commonly abbreviated as "ga.", "ga", or "G".

Calculating gauge

An n-gauge diameter means that a ball of lead (density 11.34 g/cm³ or 0.4097 lb/in³) with that diameter has a mass equal to 1/n part of the mass of the international avoirdupois pound (approx. 454 grams), that is, that n such lead balls could be cast from a pound weight of lead. Therefore, an n-gauge shotgun or n-bore rifle has a bore diameter (in inches) of approximately

${\displaystyle d_{n}=2{\sqrt{{\frac {3}{4\pi }}{\frac {1\mathrm {~lb} /n}{0.4097\mathrm {~lb/in^{3}} }}}}}$

Explanation:

• Divide 1 pound by n to find the mass of each one of the balls
• Divide it by 0.4097 lb/in³ (density of lead) to find the volume of each ball
• Multiply it by 3/4 and divide it by pi, then find its cube root (rearranged from the volume of a sphere equation) to find the radius of each ball
• Multiply it by 2 to change from radius to diameter

This simplifies to the following formula for the internal diameter of the barrel of an n-gauge shotgun:

${\displaystyle d_{n}=1.67/{\sqrt{n}}}$ (in inches), or ${\displaystyle d_{n}=42.4/{\sqrt{n}}}$ (in millimeters).

Likewise, given the diameter in inches, the gauge is

${\displaystyle n=4.66/d_{n}^{3}}$

The gauge of firearms is determined by: 1 pound/gauge = weight of lead sphere. Caliber of the bore is then measured.^[2]

Zdroj:https://en.wikipedia.org?pojem=12_gauge_shotgun
Text je dostupný za podmienok Creative Commons Attribution/Share-Alike License 3.0 Unported; prípadne za ďalších podmienok. Podrobnejšie informácie nájdete na stránke Podmienky použitia.

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1 Pound / gauge = weight of lead sphere		Diameter of bore is then measured
gauge	pounds	mm	inches
0.25	4	67.34	2.651
0.5	2	53.45	2.103
0.75	1+1/3	46.70	1.838
1	1	42.42	1.669
1.5	2/3	37.05	1.459
2	1/2	33.67	1.326
3	1/3	29.41	1.158
4	1/4	26.72	1.052
5	1/5	24.80	0.976
6	1/6	23.35	0.919
6.278	1/6.278	23.00	0.906
7	1/7	22.18	0.873
8	1/8	21.21	0.835
9	1/9	20.39	0.803
10	1/10	19.69	0.775
11	1/11	19.07	0.751
12	1/12	18.53	0.729
13	1/13	18.04	0.710
14	1/14	17.60	0.693
15	1/15	17.21	0.677
16	1/16	16.83	0.663
17	1/17	16.50	0.650
18	1/18	16.19	0.637
19	1/19	15.90	0.626
20	1/20	15.63	0.615
21	1/21	15.37	0.605
22	1/22	15.13	0.596
23	1/23	14.91	0.587
24	1/24	14.70	0.579
25	1/25	14.50	0.571
26	1/26	14.31	0.564
27	1/27	14.12	0.556
28	1/28	13.97	0.550
29	1/29	13.79	0.543
30	1/30	13.64	0.537
31	1/31	13.49	0.531
32	1/32	13.36	0.526
33	1/33	13.20	0.520
34	1/34	13.08	0.515
35	1/35	12.95	0.510
36	1/36	12.85	0.506
37	1/37	12.73	0.501
38	1/38	12.62	0.497
39	1/39	12.50	0.492
40	1/40	12.40	0.488
41	1/41	12.30	0.484
42	1/42	12.19	0.480
43	1/43	12.16	0.477
44	1/44	12.01	0.473
45	1/45	11.91	0.469
46	1/46	11.84	0.466
47	1/47	11.80	0.463
48	1/48	11.66	0.459
49	1/49	11.58	0.456
50	1/50	11.51	0.453
51	1/51	11.43	0.450
52	1/52	11.35	0.447
53	1/53	11.30	0.445
54	1/54	11.23	0.442
55	1/55	11.15	0.439
56	1/56	11.07	0.436
57	1/57	11.02	0.434
58	1/58	10.95	0.431
59	1/59	10.90	0.429
60	1/60	10.85	0.427
61	1/61	10.77	0.424
62	1/62	10.72	0.422
63	1/63	10.67	0.420
64	1/64	10.59	0.417
65	1/65	10.54	0.415
66	1/66	10.49	0.413
67	1/67	10.44	0.411
67.62	1/67.62	10.41	0.410
68	1/68	10.40	0.409
69	1/69	10.34	0.407
70	1/70	10.29	0.405
71	1/71	10.24	0.403
72	1/72	10.20	0.401
73	1/73	10.16	0.400
74	1/74	10.10	0.3978
75	1/75	10.05	0.3960
76	1/76	10.01	0.3942
77	1/77	9.97	0.3925
78	1/78	9.93	0.3908
79	1/79	9.86	0.3892
80	1/80	9.84	0.3876
81	1/81	9.80	0.3860
82	1/82	9.76	0.3844
83	1/83	9.72	0.3828
84	1/84	9.67	0.3813
85	1/85	9.65	0.3798
86	1/86	9.61	0.3783
87	1/87	9.57	0.3769
88	1/88	9.54	0.3754
89	1/89	9.50	0.3740
90	1/90	9.46	0.3726
91	1/91