STG57

[Patrick Chadwick]

...still sounds like an overall length thing, bullet in LEAD or commencement... t...

Hallo Ovidio, I agree with Jim. It is not sufficient to rely on OAL for really precise bullet seating. Variations in the ogival curve between different types and (on the sub-tenths of a mm level) maybe even between batches of the same bullet will affect the precise seating depth at which the bullet contacts the transition cone a.k.a. throat.

I suggest that you take a couple of your reloads, ink over the contact area with a felt pen, and load/unload them in the rifle by hand. If there is any marking of the inked area, then you know that your bullets are touching the lands. All mechanical processes - bullet manufacture and bullet seating - have tolerances, and if the bullets are too close to the lands (which in my view is anything under 1/10 mm) then some will be touching, some will be free, and the variations in pressure will affect the grouping.

My guess is that if you seat the bullets to be at least 0.25mm off the lands performance will be improved.

Give it a try!
Patrick