The RCBS Case Master is a great tool for about $90. Its main purpose is to measure case run-out. However, there is too much "spongy" movement as you roll your cases on the V-blocks. This mechanism is quite rickety because of the way the V-blocks are secured only to the front rail. This tool can be improved quite a bit with one simple modification. I've added a horizontal 4" x 5/16" square steel crossbar, that clamps both sliding V-blocks "down" to the base using a knurled nut. This upgrade removes all vertical play in the v-blocks, and gives very consistent readings without any unwanted fluxuation. This tool is now a pleasure to use.
These are common parts that I used for this upgrade. They can be found in almost any hardware store. You can see how these parts are oriented in the first picture (on top) of the page. For a few bucks you can remove ALL of the play in this unit, and make it much easier to use. Of course, you can buy similar units that don't require any modifications. However, most of them are more expensive, and keep in mind that the RCBS Case Master also performs other important functions.
This is one of the most overlooked features of the RCBS Case Master. It can measure brass thickness back near the case web. This is particularly helpful if you suspect the brass might be getting thinned from repetitive reloading. If you're going to get a case head separation, that's where it will happen - near the rear of the case. The gauge is zeroed on the tip of the horizontal rod. Then just slip your case over this thin rod to the area you want measured. This is far better than using the old "paperclip" method.
Measuring case neck thickness is another good feature of the RCBS Case Master. Altogether, this multi-purpose measuring tool is quite a good deal for the money. Add this simple upgrade to improve the case run-out feature, and you'll have a serious reloading tool.