wiiawiwb said:
What is the downside to reloading 9mm with a lengthy OAL
+1 to Walkalong/rodregier's comments.
Downside to loading 9mm long is reduction in neck tension and chamber pressure build, especially for 115 gr FMJ/RN bullet with shorter bullet base and particularly .355" sized plated bullets with rounded bullet base which leaks more gas and this problem gets worse with slower burning powders at lower powder charges.
Many reloaders complain 9mm is more difficult to reload accurate than other calibers like 45ACP. This may be due to smaller case volume that is more affected by reloading variables not so much noticed in larger case 45ACP.
When primer ignites powder, there are several factors to consider to produce more consistent chamber pressures which leads to more consistent muzzle velocities that results in lower SD number:
- Powder case fill/powder position in relation to primer flash
- Powder charge/load data vs powder burn rate
- Initial powder burn and chamber pressure build from neck tension/bullet seating depth
- Continued chamber pressure build from case neck/mouth expanding to seal with chamber
- High pressure gas leakage as bullet jumps through leade/freebore to start of rifling
- Max chamber pressure build from bullet's bearing surface engaging the start of rifling
With 115 gr FMJ/RN bullet, while longer OAL/COL will reduce gas leakage from shorter jump to start of rifling, it's been my experience that reduction in neck tension/chamber pressure build can overshadow gas leakage if using powder charges less than near max and/or using slower burn rate powders.
With 124 gr FMJ/RN bullet, this is less of an issue due to longer bullet base and I will often use longer 1.150-1.160" OAL to reduce gas leakage and to reduce powder charge compression but prefer to use shorter 1.130"-1.135" for 115 gr FMJ/RN bullets.
Atlanta Arms match ammo which is used by various match teams, including Army Marksmanship Unit/Marine Service Pistol team, uses 1.130" OAL for 115 gr FMJ match ammo that has accuracy of 1.5" group at 50 yards loaded to minor power factor of 125/1150 fps (BTW, 1.145" OAL is used for 124 gr FMJ) -
http://atlantaarms.com/products/9mm-115gr-fmj-match-amu.html
To increase consistency of chamber pressure and muzzle velocities, for my carbine load development with 115 gr FMJ/RN bullets, I tested 1.130" to 1.160" OAL with different powder/charges and so far, shorter 1.130" OAL with faster burn rate powders at near max/max charges are producing greater accuracy -
http://www.thehighroad.org/showthread.php?p=10245856#post10245856
With that said, sometimes you have no choice but to load longer as loading shorter will compress the powder charge. For my accuracy testing with 124 gr Berry's/RMR bullets, 1.160" OAL was used with WST/Titegroup/BE-86 to not compress the powder charge to produce sub 2" groups at 25 yards -
http://www.thehighroad.org/showpost.php?p=9924922&postcount=137