I have both gauges pictured below and they measure runout two different ways.
The RCBS case mastering gauge is the Swiss Army knife of gauges and measures runout, case wall thickness and neck thickness. The problem with this gauge is the V-blocks are painted aluminum. And over time the paint wears off and the case ends up dragging on bare aluminum. Meaning the cases become hard to rotate unless the V-blocks are lubed.
The Hornady gauge holds the case like it would be in the chamber if the case is full length resized. Meaning held by the recessed bolt face and the bullet in the throat and the case body not touching the chamber walls.
And my point being if a case does not have uniform case wall thickness the case body will expand more on the thin side of the case when fired. Meaning you will be rotating a egg shaped case in the V-blocks.
At the Whidden custom die website they tell you the most concentric cases are produced by non-bushing full length dies. And a full length resized case has wiggle room in the chamber to let the bullet to be self aligning with the axis of the bore.
Bottom line if I had to do it over I would buy the Sinclair gauge that rotates the case on ball bearings. And use this same gauge to see if the runout is caused by a egg shaped case or by just the case neck. I have Remington cases with up to .009 neck thickness variations that end up egg shaped after firing. So why buy a very expensive gauge if you don't have the best brass you can buy.
Note, the military considers match grade ammo to have .003 or less bullet runout. And a full length resized case is more forgiving with bullet runout than a snug fitting neck sized case. So remember you are checking case uniformity and quality when measuring bullet runout
I have a ball micrometer and the Redding case neck thickness gauge and prefer the Redding gauge. With one quick turn of the wrist you will see the total neck thickness variations. When checking case neck runout I first check the neck thickness variations first and then case neck runout. And want the runout as close as possible to the neck thickness variations on cases that are not neck turned. Then after the case is loaded I check the "bullet" runout as the last check.
All my rifles are off the shelf factory rifles and I see no sense in expensive runout gauges. And once I started using Forster full length benchrest sizing dies, the case neck runout has been .001 or less.
