The new generation of VLD (very low drag) projectiles have brought with them more accuracy, and more requirements to optimise. It won’t be new to shooters that bullets have a sweet spot when it comes to seating depth.
For a long time, F-Class and BR shooters have been ‘jamming the lands’ – setting seating depth out long enough where the bearing surface is actually touching (or even slightly pushed into) the throat of the rifling. Minimising jump eeks out the final bit of performance.
However, for practical shooter and hunting, having a projectile that is pushed into the lands may not be appropriate – should you not pull the trigger and need to unload the rifle, there was significant potential to leave the bullet in the rifle and end up spilling powder all through your action. Not ideal in the field.
In addition, for those who are mag loading, loading the projectile with a long seating depth may not be an option at all. For my .308 build, I have to stick with the SAMMI spec’ed 2.8 inches OAL – any longer and the projectile starts hooking up on the front lip of my AI mag. However, that isn’t to say that 2.8 is actually the optimal length for seating. Just safe for feeding.
For those of us who are able to work with single feeding, or, want to have a play within the confines of our magazine length, we need a method to hone in on the best length.
So, for most projectiles, there is going to be a sweet spot around .30 wide – how do we find it?
We could just make up a series of rounds, seating depth shortened from jamming into the lands outwards – .005 at a time. However, you are likely to burn out the barrel before your find the sweet spot. Instead, try out this methodology…
AFTER already determining your optimal powder charge (Ladder, OCW, OBT method), load up 20 rounds as follows –
- .010 off the lands (jump) 5 rounds
- .050 off the lands (jump) 5 rounds
- .090 off the lands (jump) 5 rounds
- .130 off the lands (jump) 5 rounds
Shoot in ‘controlled’ wind conditions to see how they group. One of the strings will likely outperform the other three significantly.
Now, we have our rough point to tweak from. Generally, loading up another two strings, up and down .005 will let you hone in a bit more.