Over-All Length (OAL)
- explained in detail -
I'm always finding shooters looking for a more complete understanding
of the term "over-all length" (OAL) and how to find the best length for their handloads. Most reloaders have noticed that reloading manuals show
one specific OAL for every different caliber. This OAL dimension shows the industry standard (SAAMI spec.) for a particular cartridge. Those published
loads have been tested to be safe at that specific length, and they will always fit in your magazine. However, finding the "optimum" OAL for your particular
rifle requires a bit more information than most reloading manuals provide. That's where the confusion sets in. Understanding the unique bullet
ogive (side of each bullet) will help clear things up. The bullet ogive is much more relevant than the actual OAL regarding chamber pressure.
This article explains "the rest of the story".
Factory ammo is also made according to industry standard dimensions so your handloads will fit in any rifle of the same caliber,
and their ammo will always fit in your magazine. As a result, they seat their bullets at least .100" deeper than necessary. Some rifles
(especially Weatherby Magnums) have well over .250" of free-bore (distance to the rifling). They do this to safely reach the highest velocity
possible. On most production rifles, if you seat your bullets close to the rifling, your handloads will be too long to fit in your magazine. Finding
the optimum OAL for maximum accuracy usually requires you to reduce the distance that the bullet travels before it contacts the rifling.
This also helps maintain concentric alignment between bullet and the bore, so that your bullet gets a good start down the barrel.
Setting the OAL correctly for your "particular" chamber is the cheapest accuracy improvement there is. However, increasing the
OAL will increase chamber pressure. To understand why chamber pressure is increased, think about this example of "forward momentum". A car can
easily drive over a curb with a little running start; but if you park right up against the curb and step on the gas - it requires a whole lot more force to start
moving. Keeping this in mind, it's easy to understand why seating your bullet against the rifling increases chamber pressure. When we increase
the OAL, we need to work up loads with caution, and expect chamber pressure to increase. Experienced shooters know that this is one essential step to
improve rifle accuracy. The optimum length of accurate handloads will almost always be quite a bit longer than factory loads.
We need to remember that the tip of the bullet is not what contacts the barrel. Actual contact with the rifling occurs along the
side of the bullet, so that's where the OAL measurement should be made. The picture at the right shows a "slotted case tool" that I made to determine the
exact OAL measurement. This length measurement is based on where the bullet contacts the rifling. I have one of these modified cases for each rifle
caliber that I reload. Just take a resized case, and cut 3 or 4 slots in the neck. This is an easy project with a Dremel tool using a cut-off disk.
This makes a great chamber length gauge that will grip a bullet with just enough tension to hold it in place. This becomes an OAL gauge for one specific caliber using one particular type of bullet.
I use this "split case tool" to do find the exact distance between the bullet and the rifling. With the test bullet seated in my "split case
tool", simply insert this round into your chamber. The rifling will push the bullet deeper into the case. Then carefully extract the round without disturbing
the bullet. After this gizmo is extracted, you can measure it from the bullet ogive back to the base of the cartridge. This measurement is then used to
adjust the seating depth of your dies and comparing to that measurement to your completed handloads.
The picture at the left shows another use for our Digital Headspace Gauge. It is shown measuring the "split case tool".
At this point, just zero the gauge and then measure your handloads. This gauge will then display the distance to the rifling that YOUR handloads
will have in YOUR particular rifle. In most cases a -.002" jump to the rifling is ideal. Due to the irregular point on most bullets, you can't
expect to measure a consistent OAL to the tip of the bullets. Except for a good fit in your magazine, the total over-all length is not as important. It is
more accurate to measure the over-all length of your cartridges from the bullet ogive.
Your barrel will eventually develop enough throat erosion to increase the OAL setting for your rifle. Magnum calibers (and several other
hot cartridges) usually cause considerable throat erosion after firing just a few rounds in a new barrel. However, that wear will slow down considerably after
the barrel is broken in. When testing the accuracy of handloads for hunting rifles, start experimenting with the longest OAL that will fit in your magazine.
Then start making your handloads in .003" shorter increments until you find the OAL that shoots best. I usually settle on the longest OAL setting
that will shoot well, and fit in the magazine.
If you are seeking the ultimate accuracy (and if you don't plan to use your magazine) you should start at least .001" behind
the rifling, and try reducing the OAL in .003" increments, until you come up with the most accurate load. I always avoid seating the bullet long enough to
contact the rifling, because you will eventually need to extract a loaded round. When that happens, you could find that you have a bullet stuck in the rifling.
You'll become aware of this as your case pulls away from the bullet, spilling powder all over the place.
Maximum accuracy is only possible after you've found the optimum OAL for your particular chamber. Give this method a try, and see
how much your accuracy improves.