1) Carefully push a cleaning rod into the barrel from the muzzle until you can feel the tip emerge past the chamber with your finger. Withdraw the rod ~6-8" and leave it in the barrel.
2) Place your bullet into the OAL Gauge case neck, loosen the set screw and allow the plastic push-rod to slide out until only part of the bullet nose is still visible. Tighten the set screw GENTLY...the push-rod is plastic and tightening the set screw vigorously will create a permanent dent in the groove running the length of the push-rod.
3) Push the case/bullet into the chamber, holding it firmly in the chamber against the shoulder with your index finger/thumb out near the very end of the aluminum outer rod (i.e. right where the plastic push-rod goes into it). I angle the tool with the "upper surface" at ~2:00 so the set screw is conveniently accessible. Use your other hand to gently slide the push-rod and bullet into the chamber until you just barely feel the bullet touch the lands. You can then use your index finger and thumb to continue holding the shoulder tight, as well as to squeeze them down over the plastic push-rod to prevent the bullet from backing out. Tighten the set screw GENTLY. I am right-handed, and so push the tool/bullet/push-rod with my left hand, and tighten the set screw with my right.
4) Carefully push the cleaning rod back into the barrel to move the bullet and tool out of the chamber, where you can withdraw it without the bullet falling out (if it does, it's ok to simply place it back in the case neck prior to measurement...it should be that loose). As you withdraw the tool from the back of the action, tilt it upright so the bullet doesn't fall out. Withdraw the cleaning rod back out ~6-8" for the next measurement.
5) Measure cartridge base-to-ogive and COAL with the tool tilted slightly upward...again, so the bullet doesn't fall out. It will likely be necessary to jiggle/move the lower jaw of the caliper a bit until it is perfectly flat against the cartridge base and the measurement stays constant. >>>Note getting the lower caliper jaw perfectly flat against the cartridge base is IMPORTANT when measuring with calipers. You may have to jiggle it around a bit, but when it becomes flush, you will "feel" it.
6) Repeat using the same bullet however much is necessary until you get the same measurement every time. Repetition will allow you to develop a feel for when the bullet is just touching the lands, how much to tighten the set screw, how much pressure to use holding the case in the chamber and on the plastic push-rod to keep the bullet touching the lands without jamming it. FWIW - when you use the cleaning rod to eject the bullet/tool from the chamber, you should NOT feel any initial resistance. If you do, it probably means you pushed too hard on the push-rod and jammed the bullet a few to several thousandths into the lands. So if you have to push a little harder on the cleaning rod at first, then the bullet/tool sort of "pops" loose suddenly, you jammed the bullet and the measurement will need to be re-done. If you follow these instructions exactly, it won't take very many measurements before you get that hang of it.
Caveats:
The procedure I have described is for the straight OAL Gauge in a bolt rifle, but it can also work with the angled tool in an AR-type rifle. However, the push-rod in the angled tool is much more difficult to move in/out, so it is also much more difficult to feel "touch". Nonetheless, the proper feel can still be developed with enough repetition. Likewise, the use of the OAL gauge is far easier when the rifle's freebore diameter allows free movement of the bullet out to touch the lands. For example, bullets slide completely freely out to "touching" in my .308s, which are all cut at 0.3085" diameter. However, I have a couple .223 Rem rifles that are throated long for Berger's 90 VLD. The freebore diameter there is 0.2242", which is tight enough to actually cause the bullet to stop moving at the beginning of the throat. So it is necessary to push a little harder on the push-rod to get the bullet moving again, but the be able to stop when you feel the bullet "touch" the lands. It is thus more difficult to obtain reproducible measurements with the tight-throated .223 Rem. Nonetheless, it is still possible with practice.
