Published load data COL/OAL vary because test barrels they used to measure chamber pressures have different leade (Space bullet jumps from case to start of rifling). And since most universal barrel fixtures used for testing are single shot that do not feed from the magazine, us reloaders must determine the OAL that will work in our pistols/barrels/magazines.
Load Development - First step in load development with a new bullet is determining OAL. There are two different OALs that we use, Max OAL and Working OAL.
Max OAL is the longest length chambered round's nose will not touch the start of rifling. But the Max OAL may not reliably feed from the magazine. So we function test by feeding dummy rounds (no powder, no primer) at Max OAL from the magazine and see if they will reliably feed from the magazine (Pull slide back and release without riding the slide). If they don't, I will incrementally decrease the OAL (say by .005") until they do.
Working OAL is the longest length that will reliably feed from the magazine. Often Max OAL can be the Working OAL but usually, Working OAL is shorter than Max OAL.
Determining Working OAL for Multiple Pistols - Just because a particular Working OAL works in your pistol doesn't mean it will work in another pistol as factory barrels can have different leade length, start angle of rifling, groove-to-groove diameter, different rifling type and chamber dimensions (SAAMI maximum or SAAMI mininum). Gen1 through Gen4 Glock barrels have leade length on the longer side and 9mm Glock barrels can allow longer than SAAMI max OAL of 1.169" depending on bullet nose type. In recent years (Including Glock with Gen5 "Marksman Barrel"), factory barrels have moved towards shorter and shorter leade length and thus require shorter Working OAL.
To address this new trend, this THR support thread lists max/working OALs of various factory/aftermarket barrels matched to particular brand/weight/nose type of bullets for reference -
https://www.thehighroad.org/index.p...let-max-working-oal-col-for-reference.848462/
From the reference thread, following are listed as max/working OAL for RMR 9mm 124 gr FP Match Winner (Current production with larger .074" copper fold over at base -
https://www.thehighroad.org/index.p...g-oal-col-for-reference.848462/#post-11077625):
- 1.150" / 1.150" - KKM aftermarket barrel
- 1.140" / 1.140" - M&P Shield factory barrel
- 1.095" / 1.095" - Tactical Kinetics aftermarket barrel
- 1.090" / 1.080" - Taurus PT-809
- 1.085" / 1.085" - Lone Wolf aftermarket barrel
So if you load for multiple pistols, use the Working OAL that will work with barrel with shortest leade. FYI, below is comparison picture of different RMR 124 gr jacketed bullets to show different max/working OAL depending on nose profile. OALs shown is Working OAL for my barrel with shortest leade.
Powder Work Up Consideration - Once a Working OAL is determined, then proceed to powder work up. One consideration I use when conducting powder work up is using significantly shorter OAL (deeper bullet seating depth) than published and I will reduce start/max charge by .2 - .3 gr depending on the amount of bullet seating depth.
If you look at comparison picture above with different nose type bullets, you will notice different bullet base lengths. Depending on the Working OAL and bullet type used, bullet seating depth will vary.
Walkalong makes very good point. It's how deep the bullet base is seated inside the case neck that matters more than OAL. Usually, Truncated Cone/Flat Point bullets have longer base than FMJ/RN bullets and bottom of bullet base will get seated deeper in the case than FMJ/RN bullets.
So when referencing FMJ/RN load data that used 1.125"-1.150" OAL for FMJ/FP bullets at much shorter OAL which results in deeper bullet seating depth, I will consider reducing my start/max charges by .2 - .3 gr. As Walkalong suggested, it's good idea to start low as you can always go up.
Below are load data for 124/125 gr bullet from Hodgdon (Since 124 gr bullet weight can vary by 1.0 gr, I reference load data for 125 gr bullet interchangeably with 124 gr bullet) -
http://www.hodgdonreloading.com/data/pistol
- 125 gr Hornady HAP W231/HP-38 Dia .355" COL 1.069" Start 2.8 gr (753 fps) 25,300 PSI - Max 3.3 gr (876 fps) 33,600 PSI
- 125 gr Lead Cone Nose W231/HP-38 Dia .356" COL 1.125" Start 3.9 gr (1,009 fps) 25,700 CUP - Max 4.4 gr (1,086 fps) 31,200 CUP
- 124 gr Berry's HBRN-TP W231 Dia .356" COL 1.150" Start 3.9 gr (920 fps) 27,400 PSI - Max 4.4 gr (1,037 fps) 31,900 PSI
- 125 gr Sierra FMJ W231/HP-38 Dia .355" COL 1.090" Start 4.4 gr (1,009 fps) 24,600 CUP - Max 4.8 gr (1,088 fps) 28,800 CUP
Note the different bullet types/nose profiles/diameters/COL/OAL and different start/max charges. IMO, RMR FP Match Winner comes closest to Lead Cone Nose as far as bullet nose/base and would feel comfortable referencing my initial/starting powder work up from. But as the Working OAL decreases below published 1.125", I would consider reducing my charges as I can always go higher.
And as ray15 posted, another consideration to factor is your barrel's groove-to-groove diameter and bullet diameter. Not all commercial 9mm jacketed bullets are sized the same and can vary from .354" to .356" (With .355" being typical) -
https://www.thehighroad.org/index.p...re-sized-the-same.818806/page-2#post-10567453
Many factory barrels are oversized at .356"+ and accuracy benefit from using larger sized bullets (FYI Speer TMJ/Gold Dot are sized .3555" and Zero FMJ .356"). RMR 9mm jacketed bullet sizing run slightly larger at .3555" and this can increase chamber pressure a bit and may require slightly less powder charge to produce same muzzle velocity as published.
Determining Target Loads - For my powder work up, I look for two things:
- Powder charge that will reliably cycle the slide and extract/eject spent cases
- Powder charge that will produce smallest groups (most accurate)
As I conduct my powder work up and once achieve reliable slide cycling and spent case extraction/ejection, I will look for accuracy trend to determine possible light target loads. Depending on the powder used, this may not be the case as with slower burning powders, you may not achieve accuracy until high-to-near max load data.
And if I achieve reliable slide cycling and spent case extraction/ejection with start charge and good accuracy (especially with W231/HP-38/Sport Pistol and faster burning powders), I will "work down" from start charge to see if I can identify even lighter target loads. (Disclaimer: "Work down" below published start charges at your own risk)
Determining Most Accurate Loads - After my powder work up, once the most accurate powder charge(s) is identified, especially with light target loads, I will then incrementally decrease the OAL (say by .005") to see if accuracy improves without compressing the powder charge. You can calculate max case fill by following this post -
https://www.thehighroad.org/index.php?threads/red-dot-9mm-and-case-fill.848934/#post-11078957
So even though your starting working OAL may have been 1.150", you may end up with 1.130" as your most accurate load.
These are many reasons why members post different OALs for the same bullet.
I hope this helps with your load development.