Truth is British gunmakers would give the customer whatever he was willing to pay for. It is a metric issue, or conversion by and large. A lot of Spanish guns are 27” which is 68.58mm. 70mm would be 27.56” so 27” is a slight compromise. 29” is 73.66. Some English makers used 29” to differentiate their guns being slightly lighter. 25” was the brainchild of WWGreener which is 63.5mm. The metric to inches is a bit messy.
These inches to metric conversions need to either have the decimal point moved, or change millimeters to centimeters. But as JBLondon correctly notes, English gunmakers were still measuring barrels in inches prior to 1965, when the Metric System was adopted. So the commonly seen barrel lengths of 27" or 29" have absolutely nothing to do with the Metric System, and are most likely a matter of marketing and/or inventory control. It would be interesting to know if there was some other logic behind the adoption of those uneven barrel lengths. Perhaps we will soon be informed in the Perpetual Never-ending Epic OCD Reilly Thread that it was yet another invention by E.M. Reilly and his imaginary 300 employees.
In addition, most U.S. makers were using the even numbered lengths of 26, 28, 30, and 32 inches ( or more) long before most of the world adopted the Metric System. But we see some guns that vary from those common even number barrel lengths. Robert Elliot stated that the Syracuse Lefever guns were sold only in lengths of 28", 30", 32", and rarely in 26" lengths, and that uneven lengths are a good indication of cut barrels. But I have seen many Lefevers with odd lengths of something like 28 1/8" or 30 3/16" that are very obviously uncut due to the presence of full length choke tapers and intact keels between the barrels and ribs. This practice of leaving the finished barrels a bit long is most often seen in the older rod cockers or two-hook cocking system guns, and some small hook guns. However, I have never seen uncut Lefever barrels that are some fraction of an inch under some even number. It seems like they were OK with giving the buyer a bit more for his money, but never less.
