Due to bullet variation you'll nearly always measure a few thousandths difference in overall length. With bullet variation being the typical cause of these minor fluctuations, seating to where they all measure precisely the same doesn't necessarily mean the seating depth is identical on each. What I"m saying is there's really nothing to be done, and nothing you could do if you wanted to.
Really?
There are plenty you can do to produce more consistent finished/chambered OAL/COL:
- You can use shorter working OAL (case wall gets thicker towards case base) to increase neck tension
- Resize brass separately to reduce shell plate tilt/deflection
- Reload on single stage press
- Sort brass by headstamp
- Use more consistent bullet
- Use slightly larger sized bullet
I found that sorting my brass made my OALs much more predictable. The difference in neck tension, not the case length, is the culprit.
If you are reloading on progressive press with shell plate and experience
OAL variation from different headstamp, chances are
you are experiencing shell plate tilt/deflection from different resistance while resizing brass, not from neck tension.
On a single stage press, OAL variation is limited by maximum ram travel and bullet seating stem contact with bullet's ogive.
If you want to reduce OAL variance from shell plate tilt/deflection, make sure the shell plate is not lose. If tightening the shell plate does not change anything, consider separately resizing your brass (which also allows you to inspect primer pockets/clean as necessary and hand prime/press prime cases separately). Using resized and primed brass not only reduces OAL variation but also makes progressive reloading effortless as you are only flaring case mouth, powder charging, bullet seating and crimping.
Since most RN bullet seating stems push on the side of bullet (instead of very tip), bullets with more consistent ogive will produce more consistent OAL/COL. While jacketed bullets tend to be more consistent than plated bullets, keep in mind bullet manufacturer as Speer Gold Dot and TMJ are thick plated bullets. With more consistent bullets, I can reduce OAL variance with my Lee dies down to less than .005". With RMR's in-house jacketed bullets made from new dies/machinery and more consistent lead alloy core, I can average less than .003".
BUT what's more important than finished OAL is bullet setback and chambered OAL. To check for sufficient neck tension to resist bullet setback when the bullet nose slams on the feed ramp, I measure OAL before and after I feed dummy rounds (no powder/no primer) from the magazine and releasing the slide without riding it. I prefer no bullet setback but will accept bullet setback of a few thousandths. If I measure more than .005", I consider neck tension inadequate and will investigate to make sure I am full-length resizing the brass and only minimally flaring the case mouth.
If die adjustment and minimal flaring of case mouth does not resolve the neck tension issue, check the bullet diameter as not all 9mm bullets are size the same -
https://www.thehighroad.org/index.p...re-sized-the-same.818806/page-2#post-10567453
Also, many factory barrels have oversized groove-to-groove diameter and benefit from slightly larger than .355" sized bullets which improve neck tension. When using slightly larger sized (.3555") RMR jacketed bullets and plated bullets (.356"+),
I essentially do not experience any bullet setback which when factored with finished OAL being more consistent will produce more consistent chambered OAL and bullet seating depth which will produce more consistent chamber pressures for more consistent muzzle velocities, lower SD numbers and smaller shot groups.
