The Math of the British 6 Pounder and the US M157 mm Version

I received this email from a fellow WWII glider researcher Michael Larkin. Michael is an excellent researcher and always amazes me with his skills. His explanation is clear and concise so I am posting it verbatim. This is an area that caused me some confusion, hopefully between Ben Powers article and Michael’s post this will add some clarity.

“I am following up with this note about the difference in barrel lengths between the British 6-Pounder (the Mark II and Mark III, not the Mark I and Mark IV) and the US M1 57 mm version.

The difference in barrel lengths was so-called 16”, the 6-Pounder being the shorter, so it fit in the CG-4A (to also say it had retracting trails that could fold up to fit in the glider as well. . . those long arms at the back end of the canon).

We could say that difference in the length of the barrel was actually 15.7 inches. Why?

Well, the British Mark II version (not the Mark I version) was 43 calibers in length (the Mark I, which was never put into full production, was 50 calibers in length).

But the specs that were given to the Americans to manufacture on license were based on the Mark I version, and so it had the 50 calibers-length barrel, and they stuck with it.

So let's run the math.

You get the length of the barrel by multiplying the bore of the gun times the calibers. Therefore, if we take 57mm x 43 calibers, we get a length of 2,451mm, which converts to 96.5 inches for the British 6-Pounder Mark II and Mark III.

For the M1 57mm American version, we take 57mm x 50 calibers, which gives us 2,850mm in length, which converts to 112.2 inches.

The difference between 112.2 inches and 96.5 inches is 15.7 inches! But everyone rounds it up to 16 inches.

So, when someone says these words … “the British 6-Pounder was shorter than the US M1 57mm by 16 inches”, it is ‘right’, but not ‘accurate’.

You would need to say these words to be accurate … “the British 6-Pounder Mark II and Mark III were 15.7 inches shorter than the American M1 57mm”.

If not said like that, someone could say you were wrong depending on which version of the 6-Pounder you were talking about. The British, again, changed back to producing a 50 calibers-length barrel in the upgraded Mark IV version … meaning a Mark IV 6-Pounder would never fit in a CG-4A either!”