Nobody's yet mentioned that there are 2 types of these telescoping bore
gauges. One has a fixed anvil, the other both anvils move. I find the
fixed anvil type better to use. They can be easily adjusted by tapping
the handle on a suitable object, say a toolpost, so that the movable
anvil either gives a smaller reading. Or, by turning it over, the
movable anvil gives a bigger reading. I don't think I'd trust any
measurement taken by locking the gauge while it's in the bore.
A nice old-fashioned spring caliper could be used too.
Jordan