IMHO, you need a guage that will measure to the nearest .001". I Jerry riged a simple and workable (however crude) runout guage using an old caliper, 4 quarters, epoxy, screws and a few pieces of scrap wood. I started using the caliper that I had and simply installed it on the jig when I wanted to try to measure some runout. A couple months later, I dropped the caliper and I noticed problems with the threads on the caliper right where I needed it to measure a 300 wsm
cartidge and I had to replace it so I used that one to perminantly mount on my jig. A caliper is $30 so I wouldn't recomend doing this if you have to purchase a caliper for that specific reason - just get a RCBS guage for $70 and be done with it.
This thing works well enough for me ;)
I'll attempt to download a picture of my little piece of cra..... "art" ;)