Shallow Bore Test Gage

[Toolmaker51]

Liking the idea of a rail based device.
I often use 'Indi-Cal' which is a great little tool, provided you've a ring gauge or sample target part. A little tedious with micrometer, I usually start with a vernier/ digital caliper, being a member of the 4:1 (tolerance:resolution) crowd.
The rail makes repositioning less iffy.