I like the Sinclair just fine. I can't say it is the best because I have never used all of them. I can say that it suits my needs and I have had no reason to change. A concentricity gauge and a bore scope will give you a education for sure. They helped me up my game by a lot.
After looking at the Hornady and the RCBS gauges I went with the Sinclair concentricity gauge which works extremely well. One piece of advise I was given is to get the dial gauge rather than the digital gauge. It is much easier to watch the needle move rather than watch LCD numbers go up and down. 21 Century also make a nice unit.
I use the Forster Co-Ax Case Inspector and find it works great for my applications, but I'm just a simple "back-yard" enthusiast. The aforementioned gauges I'm sure are great examples of precision instruments. I own a couple little gadgets from 21st Century and John really knows his stuff and has top-notch customer service.
Below the Hornady gauge supports the base of the case and the bullet like the top illustration. This would be like a chambered full length resized case and the body of the case not touching the chamber walls.
The bottom illustration spins on the case body and if the case is not perfectly round it will add to the measured runout. If the case is thinner on one side when fired it will become egg shaped. So when checking runout it could be caused by the case body. This holds the case more like a neck sized case where the case body has a guiding effect on the bullet and the bore. And this type gauge will give twice the runout reading than the Hornady gauge.
Lol. That about covers all of them... Pretty much what I expected though. It seems as if everyone basically like the ones that they have. So maybe if we approach it from the other end... anyone use one that they didn’t like and ended up replacing?
If I had to do it over I would have bought the Sinclair concentricity gauge. The RCBS gauge has painted aluminum V-blocks and when the paint wears off the cases become hard to rotate.
The Sinclair gauge has ball bearings that the case rotates on with far less effort.
I would also recommend the Redding neck thickness gauge, and with one twist of the wrist you know how uniform your necks are. I have had Remington .223 cases with over .009 thickness variations so its good for sorting cases. It also helps if your are checking your cases after sizing and checking case neck runout when not neck turning. Meaning you subtract the neck thickness variation from the runout reading for actual runout.