Inside -- Outside Calipers
calipers allow for a quick visual check of measurements like the
distance between a barrel channel and ramrod grove.
These are brass and date from the first half of 20th-century.
In the John Wyke of Liverpool tool catalog, from the third quarter of
the 18th century, these are listed as being made of steel. He offered
Cross section of a rifle forestock. The calipers could be used to
quickly measure from the bottom flat to the outside of the stock.