This morning I set about loading a batch of ammo for the Weatherby Vanguard Varmint rifle I am reviewing. Previous testing at the range with untuned handloads and factory ammo showed that rifle shot quite accurately. The 55 grain Nosler Ballistic Tips in particular shot about 0.5 MOA. That sort of performance deserves to be followed up with some handloads tuned to the rifle. The starting point for that is to determine the seating depth where the projectile just touches the rifling lands.
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.
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.
Take a marking pen and colour all the straight sides of the projectile. 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.
The projectile will clearly show the scrape made in the ink marking as it was pushed back into the case on chambering. 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.
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 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. Once you know the seating depth where the projectile just touches the lands you can then test different seating depths, seeking the most accurate position.
With hunting rifles it is good practice to always seat the bullet so that it is clear of the lands. I start with my first test batch at 10 thousandths of inch short of the lands. From there I reduce the cartridge length (i.e. seat the projectile deeper in the case) in steps of 20 thou. Once I have found the batch that grouped the best, then I adjust about that in 5 thou steps to find the best position.