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To use the OAL, thread the modified case onto the end of the
gauge. In the above photo, I have screwed a .223 case onto the OAL Gauge.
I've also slipped the bullet into the neck of the case. For this
measurement, I am using a Nosler 55 grain Spitzer Ballistic Tip bullet.
You can just see the polymer tip peaking out of the end of the case.
This is a Remington 788 .223 rifle I've had for many, many years. After removing the bolt, I carefully slipped the gauge into the chamber. Note that you don't want to let the bullet drop out of the end of the case. You will probably need to tip the muzzle up in order for the bullet to stay in position.
With the modified case now fully seated in the chamber (by applying gentle forward pressure on the red aluminum body), the plunger rod is pushed forward using a small amount of pressure until resistance is met. The resistance one feels is the bullet ogive engaging the rifling. Maintain pressure on the rod while you lock the thumb screw at the rear of the OAL Gauge body.
Slide the OAL Gauge out of the receiver. The bullet will
most likely stick in the rifling....that is OK. Using a wooden dowel rod
or similar inserted at the front of the barrel, gently dislodge the bullet from
Gently place the bullet back into the end of the modified case. Without loosening the thumb screw, place your dial caliper as shown above. Note that there is an area on the aluminum body that is cut away just for this purpose. Adjust your dial calipers to measure the overall length of the modified case and bullet.
The above measurement is maximum overall length for that particular bullet in that specific rifle.
If you stop and think about it for a minute or two, there are a couple of things that can influence the accuracy of the measurement you just made.
My suggestion (aside from starting with a clean rifle) is to repeat the OAL process about 6 to 10 times and note the final OAL measurements on a piece of paper. Once you begin to consistently apply the same pressure to the gauge body and plunger rod, the subsequent max OAL measurements should be coming out very close to each other....within a couple thousandths of an inch of each other.
DO NOT let this write-up be a substitute for your reading the documentation that Hornady supplies with the OAL Gauge. Part of my motivation to put this write-up together was to give others an idea of how this maximum cartridge overall length measurement can be obtained. I had to do some digging via Google before I found the info I was looking for about this gauge. Perhaps this write-up they find themselves standing in front of the Hornady display trying to figure out if they should spend their hard earned $$ on this particular item.
There is not magic guarantee that just because you measure your free-travel and change your bullet seating depth your groups will shrink by a factor of 2 or 3. However, combining this information with a good reloading regime should help you squeeze everything you can out of your rifle and your reloads.