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Somewhere along the way, a reloader discovers that bullet groups on a target can usually be improved by adjusting the seating depth of the bullet in the cartridge. By optimizing this seating depth, one can find the best distance for the bullet's "jump" from the cartridge to the rifling. Just a few thousandths of an inch change to the distance between the bullet and the rifling can take a good group and tighten it into one that you'll be bragging about to your friends at the next club meeting.
So how do you measure this distance and apply this info to your reloading
process? Let's take a look.
The above diagram shows us that "jump" distance we are attempting to regulate. it is also commonly referred to as free-travel. The ogive on the bullet is that part of the bullet that will first engage the rifling once the cartridge is fired. When a different bullet is used (different weight and/or different manufacturer), one must make the check again to determine free-travel for that specific bullet. You do NOT want to use bullet seating depth numbers for a different bullet without first determining what they need to be. If the overall length is too long, your bullet could end up touching the rifling once the cartridge has been chambered. If the trigger is then pulled, this can easily lead to an over-pressure situation.....not good!
While the numbers vary as to what is considered acceptable
free-travel, Hornady states that values in the range of .020" to .040" are
typically used by shooters. There are other methods by which to accurately
and safely reduce the free-travel to below .020" however, the equipment being
covered here is not designed to manage that process. Hornady does not
recommend using this method for free-travel values less than .020".
There are several tools to help you determine the correct seating depth for your bullet and rifle. This write-up takes a look at the Hornady OAL (Overall Length) Gauge. With just a little practice (that means about 5~10 minutes of repeated use of the gauge), you should be able to get repeatable measurements that are within just a few thousandths of each other.
The main component of the OAL Gauge is the red aluminum piece
shown in the above photo. This particular model (shown) is used for bolt
action rifles and others, such as an AR-15, where you have unobstructed inline
access to the chamber. Hornady also makes the OAL Gauge designed to access
the chamber of a semi-auto or pump firearm where the measurements must be
conducted via the receiver ejection port. The aluminum body has a plastic
plunger rod inside of it. This rod will be used to properly position the
bullet (at the appropriate time).
There is one other important component that is used with the
gauge and that is a caliber specific "modified case" that is available from
Hornady. If you have a .308, a .223, and a .270 rifle, you would need a
modified case for each of the three cartridges assuming you were going to check
OAL for all three rifles. Luckily, the modified cases from Hornady are not
that expensive....the .223 case I bought was $5 at Cabela's. They list
most of various modified cases in the $5 ~ $6 range on their web site.
In the above photo, you can see that this modified .223 case (tapped by Hornady), can be screwed onto the gauge prior to it being inserted into the rifle chamber. This will secure the cartridge in the proper position while taking the free-travel measurement.
The base threads are not the only modification done by Hornady to this cartridge. Aside from the threads in the base, the neck is .002" over sized to allow your bullet to easily slide in and out of the case. This, as we will see, is essential for proper operation of the OAL Gauge.
Now that you know how to assemble the OAL Gauge, let's take a look at how it is used in the rifle.
More OAL Gauge