I am in the process of building me a model of the 100 mm 2A29 anti-tank gun. The real gun has a caliber length of 61. This means that the barrel is 6100 mm. Does L/61 mean the lenght from end of the breech to the muzzle brake?
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It’s the length of the gun tube, the bit that the shell fits tightly into to be propelled along. So it measures from the forward end of the breech to the outer end of the barrel, excluding anything added like a muzzle brake.
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Length of the barrel, breach to muzzle, expressed as a multiple of the bore diameter.
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…and to add to the fun, Caliber is often used to denote the bore diameter in many weapons, the M2 HB Machine gun is commonly referred to as a “50 caliber” , referring to it’s .5 inch bore, has nothing to do with barrel length.
Confused yet? 
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Important addition here: the above applies to American and British guns. It does not to German ones. German guns, at least in the Second World War, were measured in calibres from the rear face of the complete gun (minus recoil guards etc.) to the muzzle without the muzzle brake, if present. Basically, it measures the total length of the actual gun itself.
For example:
(source, and see from there for a discussion)
Total gun length of this 8,8-cm-KwK 43 is shown as “6300 = L/71”, and 6300 mm ÷ 8.8 cm = 71.59. From this follows that you can’t just multiply calibre by barrel-length-in-calibres to get barrel-length-in-centimetres (which is only to be expected, of course).
The length of the barrel alone is 6010 mm, so in British terms this is actually a 6010 mm Ă· 8.8 cm = L/68 gun, not L/71.
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Jakko, interesting info, didn’t know Germans calculated it differently. Learn something new every day, thanks.
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Just like “bore diameter”, the definition depends on the nation doing the defining.
KL
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I know from my time in the navy that caliber is defined as the length of the barrel, from the end of the chamber (or beginning of the rifling if it is a rifled barrel) to the muzzle. It is used in the mathematical ballistic formulas somehow to come up with the firing solution. Thank God my my job ended when the gun mount went BOOM!
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