- DECREASING OAL/COL TO INCREASE NECK TENSION/INITIAL CHAMBER PRESSURE (9mm FMJ/RN SPECIFIC)
Blue68f100 said:
I've had only had 1 gun (BHP 9mm) in which the OAL had any impact on accuracy. With it going to a shorter OAL reduced the group by 1/2.
This may go against the notion of longer OAL reducing high pressure gas leakage but IMO very pertinent to 9mm, especially with shorter bullets or bullet types with shorter bullet base/bearing surface.
Bullets with substantial base or bearing surface (part of bullet that engages the rifling), like 45ACP 200 gr SWC and 40S&W 180 gr TCFP, can be seated to longer lengths without significantly decreasing the initial chamber pressure build for efficient enough powder burn (of course, this is my notion only so we can discuss it further if you like). BTW, I load 40S&W 180 gr TCFP longer than SAAMI max of 1.142"-1.155" depending on the bullet nose profile.
With 9mm 115 gr FMJ/plated RN and 124/125 gr lead RN with "stepped" nose (see picture below right of Dardas/ZCast bullets), this is an issue. Due to shorter bullet base, when these bullets are loaded long (say SAAMI max of 1.169"), IMO there may not be enough neck tension to create sufficient initial chamber pressure build up, especially if the expander is oversized or too much flare is used particularly with thinner walled cases. I have verified this by measuring bullet setback from feeding/chambering from the magazine when these factors "stack" on top of each other.
IME, using shorter 1.135" for 115 gr FMJ/plated RN and even shorter 1.125" for 124/125 gr "stepped" RN increases neck tension thus improve initial chamber pressure build up. I think this is the reason why "non-stepped" RN bullets such as MBC 124 gr RN (SmallBall) are sold instead of the stepped RN. In the picture above, you can compare the difference in bullet base/bearing surface of MBC RN vs Dardas/ZCast "stepped" RN.
Winchester 115 gr FMJ loaded to 1.135" showing "bulged case neck/Coca Cola bottle look" of tapered 9mm case to indicate good neck tension (cases full-length resized with Lee carbide die).
For my barrels, MBC RN needs to be seated to 1.080"-1.100" which puts substantial amount of bullet base inside the case neck for greater neck tension (remember, 9mm is a tapered case with thickening of case wall further down the case neck) not to mention longer bearing surface riding the rifling with bullet weight placed more towards the base (shifting of center of gravy towards the back like JHP bullets bullseye match shooter prefer for accuracy) for perhaps greater rotational stability in flight. If you want greater neck tension, it's hard to beat SWC with even more substantial bullet base.
MBC RN seated to 1.080" compared to 115 gr FMJ/125 gr CN/12 gr SWC
So is this the reason why many complain 9mm 115 gr bullet is less accurate than 124/125 gr? I think so, especially if you factor in mixed range brass with old brass that has less malleability from work hardening. BUT not all is lost. If you like the 115 gr bullet weight, Berry's MFG makes 115 gr thick plated bullet with hollow base that extends the bullet base/bearing surface. I have done comparison range test and I actually got smaller shot groups from 115 gr HBRN-TP bullet than 124 gr HBRN-TP in the 9mm Herco thread (I am still scratching my head over this one) -
http://www.thehighroad.org/showthread.php?t=745656
Berry's 124 gr solid base, 124 gr HBRN-TP, 115 gr HBRN-TP and Winchester 115 gr FMJ
BTW, Dardas Cast Bullets offers both stepped 125 gr RN and non-stepped 124 gr RN bullets and offers 5% THR discount with
HIGHROAD code -
http://www.dardascastbullets.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=dardas&Category_Code=9
Missouri Bullet Company offers non-stepped 124 gr RN (SmallBall) lubed/Hi-Tek coated and offers 5% THR discount with
THR-Original code -
http://www.missouribullet.com/results.php?category=5&secondary=8
Rocky Mountain Reloading offers 115/124 gr HM plated RN with harder 11-12 BHN lead alloy core with thicker plating (.012"-.014" vs Speer TMJ at .015") called "Hardcore Match" bullets at probably the lowest thick plated prices also with 5% THR discount with
thehighroad5 code (Free shipping on all orders) -
http://www.shop.rmrbullets.com/9mm-355_c12.htm
Powder Valley and
TJConevera carry Berry's 115/124 gr HBRN-TP bullets (TJConevera with free shipping on $50+ orders).