One problem I have with my plastic calipers is that if I close the jaws all
the way & hold them against a lamp, I see light seeping thru in an uneven
way, indicating that the jaws don't have completely flat or even surfaces.
This introduces an error of about 0.1 mm, depending on where a shell is
placed. I don't know if steel models are less likely to have this problem.

Aydin