Apologies if this has been covered before. If it has, just point me at the previous thread please. I am sure someone is going to point out the bleeding obvious that has escaped me. That's why i'm asking so my soul can be filled with that beautiful 'eureka moment' when the answer is gently lowered into my lap. I realise my lack of good general education will probably account for this serious gap in my general knowledge - but i'm happy to risk ridicule among you chosen few just so i know.

SO:

ACCORDING TO WIKIPEDIA:
"The 5.6 mm caliber or .22 caliber, is a small, extremely common size of ammunition, fitted to firearms with a bore diameter of 5.6 mm (0.22 in). It is the most common bore for rimfire ammunition, and has gained popularity in the air gun discipline as a hunting/field target/HFT pellet caliber."

WHY? Why have we ended up with 0.22 and (subsequently) 0.177 as the main calibres? [with no disrespect to others intended]

I have always assumed that the derivation came about as some form of logical starting point within the scheme of Imperial measurement.
Because, after all, at some point - somewhere - mass manufacturing techniques would have needed an imperial equivalent to run lathes, and mills, and all the other lovely post-industrial-age monsters that churned out armaments. I believe i've seen quotes and articles that attribute BSA's capabilities (from everything from early air rifles to military production barrels) as capable of tolerances measured in 100,000th of an inch. Frankly, even 10,000th inch of an inch is impressive. And I'm sure BSA was not unusual when compared to other manufacturers such as remington, winchester, mauser, etc.

But again (seriously)... Why? What's the 'link' that is eluding me which meant someone (somewhere) suddenly said:

" Hey guys. What we need is a new calibre - so why don't we call it '0.22' ?
Oh: and then we can arbitrarily [?] invent 0.177 as well,
and eventually we can even refer to '0.22' as 5.6mm "



FOR EXAMPLE:

Giffard adopted '8mm' as a standard - and at least that equates to 5/16 inch (0.314 inch i think).

We all refer to 0.22 as 5.6mm, even though 5.6mm is actually 0.220472 of an inch.

But: 5mm translates as 3/16 inch, and 6mm is a 'gnats todger' less than 1/4 inch (or 4/16).

And it gets worse: 7/32 inch = 0.21875 inch (ie too small)
and yet 15/64 inch = 0.234375 inch (ie too large)

- - - - -
SO: how did we end up with 0.22 and 0.177?

(and apologies if i've got any errors in the above but that is a lot of number stuff and i had to use all my fingers and toes!)