I have three different brands of 9mm 115gr RN none of which are consistent enough in size/shape/profile/whatever (even within own brand) to simply adjust the seater and forget it.
If I adjust seater to 1.135" that's what I want. Not just close. Only one brand even approaches it but even still not close enough--I have to readjust seater for each and every round to get it right. Someone's bound to say "I wouldn't worry about it." I'm not. I'm just providing some real life examples.
Your testing results mirror my experience.
OAL/COL and resulting bullet seating depth consistency or variance have been myth busted/confirmed on several threads and dependent on following variables:
- Bullet length consistency
- Bullet nose profile/ogive consistency
- Use of mixed headstamp brass
- Bullet tilting during seating
- Shellplate tilt/deflection and inherent "play" of press
- Neck tension and bullet setback
Neck tension and bullet setback / Use of mixed range brass - Your testing didn't check for bullet setback but when slide slams bullet nose on the feed ramp, there could be significant bullet setback. We can fuss over "
FINISHED OAL and bullet seating depth" all we want but what really matters is "
CHAMBERED OAL and resulting bullet seating depth"
AFTER experiencing any bullet setback as
CHAMBERED OAL is what will actually determine chamber pressure consistency that will lead to muzzle velocity variation (SD number) along with powder charge variance (Can you say "tolerance stacking"?
) and if shot at long enough distances, bullet drop variance will result in vertical/diagonal elongation to expand group size.
Bullet setback, especially "tolerance stacking" on top of OAL variance from using mixed headstamp brass, could overshadow other reloading/shooting variables and IMO are two most important factors that can affect chamber pressures for high pressure 9mm with small internal case volume (I would say powder charge variance is next factor).
Because of these, when testing my loads for accuracy during powder work up, I always check bullet setback. I used to just push on the bullet against the bench top but this does not accurately replicate impact force exerted when slide slams bullet nose on the feed ramp. So now I test bullet setback by loading dummy rounds (no powder, no primer) in the magazine and release slide without riding it. Measuring OAL before/after will show amount of bullet setback. I prefer no bullet setback, especially for accuracy. But if I see bullet setback, especially significant setback more than .005", I will consider using slightly shorter OAL/deeper seating depth, thicker case wall brass or larger sized bullets (There are NATO spec bullets sized .354" and more common sized .355", RMR/Speer GDHP/TMJ are sized .3555" and Berry's/Zero bullets are sized .356") until bullet setback is eliminated.
BTW, neck tension and bullet setback tested on this thread (Including .354" sized NATO spec bullet) -
https://www.thehighroad.org/index.p...neck-tension-and-bullet-setback.830072/page-4
9mm bullet diameter compared in this thread -
https://www.thehighroad.org/index.p...re-sized-the-same.818806/page-2#post-10567453
OAL variance by headstamp brass myth busted/confirmed in various threads like this one from .005" down to .001" -
https://www.thehighroad.org/index.p...e-consistent-oal-on-progressive-press.921633/
Bullet length/nose profile/ogive consistency - I have found different brands/manufacturers produce different lengths of bullets and nose profile/ogive to produce different OAL with measurable variance even though same die settings were used.
Below comparison picture shows (From left to right) various 115 gr FMJ RN from Zero, RMR, Winchester (with skirt base/sharp rim), Everglades (dished base) and Federal (hollow base) - Notice hollow base FMJ produces significantly longer bullet base (close to 124 gr FMJ flat base):
I also found jacketed bullets to have more consistent length and nose profile/ogive than plated bullets. Especially for RN, bullet nose profile/ogive consistency is important as most seating dies don't push on the tip rather further down on the "ogive". So if nose profile/ogive is not consistent, then your OAL/seating depth won't be consistent.
For my USPSA match shooting in the 90s, Montana Gold jacketed bullets were used for consistency in bullet weight/length but now I use RMR in-house jacketed bullets as my reference bullets. While MG jacketed bullet weight variance was less than 1.0 gr, RMR jacketed bullet weight variance is less than 0.5 gr with greater consistency in bullet length/nose profile/ogive. I do believe these factors were the reason why ELEY chose to use RMR for their centerfire match ammunition of 9mm minor/major/PCC -
https://www.killoughshootingsports.com/9mm-ammo-c19
Bullet tilting during seating - Bullet tilting during seating is indicated by case neck bulge at bullet base on one side instead of even bulging around the case neck and can affect finished OAL/neck tension/bullet setback. Most reloaders addressed this issue by using stepped "M" style expanders and Lee Precision resolved this issue with stepped "M" style powder through expanders in essentially all of their dies/sets -
https://www.thehighroad.org/index.php?threads/dillon-550-vs-forster-co-ax.927598/post-12827974
Shellplate tilt/deflection and inherent "play" of press - Depending on the press used, especially when progressive press is used, "play" of turret or shellplate tilt/deflection can add to tolerance stacking of finished OAL variance. For some presses with this issue, using pre-resized brass will address/eliminate this reloading variable for more consistent finished OAL.
Y/D
