by Wapiti » 28 Apr 2026, 7:26 am
Mate if your gear is consistent enough, you can do your seating in .010's if you want.
You can even try to just kiss the lands as a way to centre the projectile perfectly and avoid any possibility the point gets the opportunity to enter even slightly off-centre.
Some people get one-hole groups by pushing into the lands hard. Obviously, they have adjusted their powder charge down to compensate from the steep pressure curve this generates.
Forget doing this is you expect to unload that cartridge and not fire it, because the bullet will stay jammed in there and powder will go everywhere and end your day at the range.
This is exactly the same as a countersunk-head screw dropped through a hole with a countersink cut in it, effectively exactly centring it.
But for this to work, your case has to be exactly centred in the chamber it sits in, and this is achieved by the shell itself being a nice fit lengthwise, so that the bolt-face can push and centre the shell into the counterbore that the tapered neck of that case has in the chamber.
The same way that screw centres in its tapered, counterbored hole.
I hope that makes sense.
That above is in your sizing regime and to get the most from seating depth differences, you have to take out this variation as well, at the same time.
Sounds fussy and complicated, but in no way is it.
Think of it like this... if the projectile enters the lands dead centre, because it's still held in the case neck that's dead-centre in the chamber as it does, dead centre to the bore-line (hopefully, that the whole thing is in a dead straight line) then that's the best you can do.
Another thing that people don't ever think of is that, the practice of seating the projectile in or out further, consistently, slightly changes the pressure/burning curve as well. This can find you a "sweet spot" in your group size.
You can burn up a lot of components trying to find this point, but it is one of a few points there that will do this for you.
I did find in my 338 Lapua that 10 thou from the lands worked the best with the powder and projectile I eventually settled on.
On cases I partially FLS deliberately (not neck sized only, even though I use bushing dies exclusively in guns I want this performance in) to give me the perfect length for a perfectly centred, slightly "crush" feel on the bolt handle.
Remember, a partially FLS fired case grows fractionally longer in the die before the shoulder is bumped back and you make it too short.
I think Nick Harvey explains this in great detail in his loading manual, I suggest you buy one and read it.
I tried 10, 20, 30 thou etc jump, why? Because I had to settle on a number, one I could at least be as consistent as my gear would allow and try it.
Because that's all you can do.
"The only way to avoid criticism is to do nothing, say nothing, and be nothing."
Aristotle.
Regards G,
AKA Dr. Doolittle