I'm certainly not providing any information here that most .22 rimfire fans don't already know, but being a tight-wad when it comes to purchasing a tool that's easily made, just makes me shiver. .22 rimfire cartridges headspace on the rim. Nothing knew there for most everyone here. How do we measure the rim thickness of a .22 Long Rifle round, or any of the shorter .22 rimfire cartridges? This is the cheapest, yet most accurate method I could come up with, as long as you have a caliper, or a micrometer for measuring. A .22 caliber "fired", centerfire, cartridge case, .222, .223 or a .22-250, will allow a live .22 rimfire round to slide right into the case mouth so that the rim rests against the front end of the fired case. I trimmed the case mouth true in my case trimmer and then face the cartridge head flat and smooth in my lathe. Now, that's being a bit more "anal" than it needs to be, so don't worry about the rear end of the case being perfectly perpendicular to the case body if you don't face it off. Next thing we need to do, is "zero" your caliber or micrometer on the over-all length of your fired case, like so: We now have a zero-point to work from. Next, slide the .22 cartridge you want to measure into the case mouth: Then just measure over the bottom face of the .22 cartridge to the rear end of the centerfire case and you have the rim thickness. You'll be surprised as to the varying lengths you'll get for .22 rimfire thickness. There are times when I have found Ruger 10/22 & 77/22 bolt faces that measure 0.045 to 0.050-inch deep. That's where headspace is controlled on these guns, along with the Ruger Mark pistols. Now, in many instances, accuracy will be improved in the Ruger 10/22 & 77/22 rifles when the headspace is adjusted slightly so that the firing pin can smack that rim in a more positive manner through a shorter travel. Caution is always in order when doing this sort of work on a Ruger 10/22 so that headspace is not shortened to the point where "slam-fires" will occur. Try segregating the .22 cartridge rim thicknesses in groups where the rims are very close in thickness and see if your groups improve. Can be very interesting. A safe bet is to maintain at least 0.0020-inch between the bolt face where the cartridge rim rests and where the front face of the bolt rests against the back face of the barrel. If you're not interested in adjusting the headspace, it can be an interesting project just to measure some of the .22 rimfire rim thicknesses that you have, and do it in a cheap but accurate manner.