How to Determine Overall Cartridge Length
The overall length of a cartridge (OCL) is determined by how far the projectile is seated into the case. This also has a big impact on the accuracy of the loaded ammunition. The first step in deciding on any particular OCL is to determine the bullet seating depth that results in the projectile just touching the rifling lands (JTL) in the barrel when the cartridge is chambered.
There are a variety of ways this can be done. I have adopted the following method because of its ease and simplicity. I was shown this by an experienced reloading buddy who tells me he acquired it from a Nosler reloading manual quite some years ago.
How to determine the JTL – the projectile seating depth that just touches the rifling lands.
Take a fired case, not yet resized, from the rifle that the load is being developed for. A projectile will slide freely in and out of the case neck.
Create a flat spot by pressing the case neck against a hard flat surface. This will provide just enough grip to prevent the projectile sliding freely.
Seat a projectile only a short distance into the case, say 1/8th of an inch, about 3 mm.
Chamber the round and close the action. Do not pull the trigger.
Gently open the action and carefully remove the case.
Note that projectile may remain in the barrel, gripped by the rifling. If that is so, then gently insert a cleaning rod from the muzzle and push out the projectile without damaging the tip. If the projectile is still seated in the case, then remove by hand, being careful not to rub off the marker pen ink.
Replace the projectile in the case neck and carefully push, or tap, the projectile back into the case until the end of the scrape lines up with the edge of the neck.
Using callipers, and assuming the projectile tip is undamaged, measure the overall cartridge length (the OCL) which in this case is the JTL.
Take a neck sized, unprimed case and adjust your projectile seating die to reproduce exactly the same OCL with the same projectile type. This second, unprimed dummy round with its firmly gripped projectile is now your reference JTL for that particular projectile in that particular rifle.
You will need to repeat steps 1 to 10 for every different type of projectile that you intend to fire out of that particular rifle. The JTL will be different for different projectiles.
Varying the OCL for optimal accuracy