One of the most important factors in achieving accurate hand loads is bullet seating depth. Seating depth affects not only the accuracy of your load but also the pressure and velocity and in some cases it may also have an influence on velocity spread.
So the question is: what is the correct projectile seating depth? Well, firstly it’s different for every rifle - and even rifles of the same make and calibre will shoot better or worse with different seating depths. Projectile shape also affects accuracy. Traditional Tangent ogive projectiles like Sierra Match Kings are less sensitive to seating depth than Secant ogive projectiles and VLD bullets. VLD’s are really just very aggressive Secant ogive projectiles and because the curve of a secant ogive projectile meets the parallel part of the bullet quite abruptly it doesn’t self-align in the throat as well as a Tangent bullet. Now both styles can shoot just as accurately, but one is more sensitive to seating depth than the other. Berger make a projectile called a hybrid and this has a long Secant nose that then becomes a Tangent just as it meets the parallel part of the projectile. The Tangent portion provides a better alignment into the bore while the Secant design allows for a very long nose, which gives a high ballistic co-efficient.
While the above may be a bit confusing, the upshot is that to get the most out of your rifle you should pay some attention to your seating depth. And the best place to start is by choosing a load that already shows some potential in your rifle; seating depth changes may not make a poor shooting load into a tack-driver, but a change in seating depth may make a good load great.
We need to establish the maximum overall length of the round first. There are a few ways to do this, and a few tools that make it easy. The ‘no-tool’ way takes a bit more effort, but here is how you do it. Take an empty case that has been fired in your rifle and make sure a bullet will slide into the neck easily. Now take the case and on a hard surface push down on the neck so it pushes in a fraction. What you want is for your projectile to be able to move with a little bit of finger pressure but not be seated tightly, as it would be if you resized the case. Now insert the projectile a small amount and double-check that it can move with some finger pressure. Next, get a marker pen (black is best) and colour in the bullet as it sits in the case. Insert this round into the chamber and slowly close the bolt all the way. The projectile will be pushed back into the case as the front of it contacts the rifling. Open the bolt carefully; if the projectile is stuck in the barrel tap it out with a cleaning rod, and insert it back into the case using the scrape line on the bullet to set the depth. Now you can measure your MOAL (maximum overall length) for this projectile. Use your callipers to measure the round and this will be your MOAL. From here you can determine how far off the rifling you want your bullet to be.
There are some tools like the Hornady OAL length gauges, the Redding instant indicator, the RCBS precision Mic and the hand loader’s “must have” - the Sinclair Hex bullet comparator - that are very useful; these tools can make the process of determining OAL easier and I have personally used all of them and can recommend them. The key with these tools is that instead of focussing on MOAL they give you the much more important Cartridge Base to Ogive length (CBTO). CBTO is the measurement from the base of the cartridge to the point on the bullet where the ogive starts. That measurement is where the bullet actually contacts the start of the rifling. This length is essentially the same no matter what bullet you use, and is more exact as it’s not affected by the variations that exist in each projectile due to tip and meplat variances. While the method for determining MOAL as described above will work, I would recommend using a bullet comparator so CBTO can be used instead of MOAL. So - now we have a MOAL or CBTO, what distance do we use?
When choosing your depth, one of these five options will give noticeably better accuracy than the others:
Points to note: