SH:
Early bore-guns were often short, some only 20 inches, then came 'express' rifles with long barrels, 28 inches would be typical but some were 30 inches. Nitro doubles are usually in-between, 24 inches min to 28 inches max, with most at 26 inches.

Please appreciate that this is a simplification: with the British gun trade at least, the customer usually got what the customer wanted, so there were no hard-and-fast rules!

Marrakai
http://www.hotkey.net.au/~orrs/Marrakai