Caliper for group measuring

[cpttango30]

I am looking for the caliper that is used to measure group size. The one I seen was a dial caliper with a clear plastic section on it with circles and cross hairs in them. Does anyone know where i can get one or the plastic attachment for a caliper i already own?

Thank for the help.

 

[AlNyhus]

R.W. Hart used to sell these. They are in Nescopeck, Pa.

1-800-368-3656

 

[vlcakc]

Sinclair. See links.