Increasing cartridge oal

[Catpop]

Am I correct in that increasing cartridge oal (decreasing bullet seating depth) will decrease chamber pressure (assuming bullet not touching lands)?

 

[MtnCreek]

Pressure can increase as you shorten the gap between bullet and rifling. If you're a ways off the rifling, then that's not an issue, but as you get close (like inside of ~ 0.050), pressure can increase.

You're correct that longer seating depth increases case capacity and reduces pressure.

 

[rcmodel]

I can make a lot of difference in a short semi-auto handgun round.

Not much difference at all in a long bottleneck rifle caliber.

The percentage of case capacity the bullet base takes up is much greater is an auto pistol caliber then it is in a 30-06 or something.

rc

 

[gamestalker]

I'm a die hard seat up close to the lands type when referring to bottle neck cartridges. And when I start getting to less than .020" from the lands, I begin to see a significant, although consistent, increase in operating pressures. So for this type of seating style, I always work up from no more than mid range. Once I've established my seating depth, I then begin incremental increases in the powder charge until I've reached my desired load. This can get some what cumbersome, but it really helps to avoid sudden and undesirable pressure spikes.

Loading for auto loading handgun cartridges is a bit different. I like to use the "plunk test", sort of. What I actually do is use the barrel and then rotate the bullet with my fingers to be sure I'm not touching the lands to establish maximum. Once I've established that maximum, I find magazine fit and generally go with the longest functional OAL, but I don't try to get as close to the lands as possible. And since I load with jacketed bullets and slow burning powders, I keep oal to a depth that will accommodate the heavy powder charges, in terms of pressures.

GS

 

[Catpop]

First test of three five shot groups in 1911 Kimber 45 acp 230 grain plated Rainer, coal 3.117 (just short of lands), with 4.0, 4.5, and 5.0 grains of Red Dot did not reveal my hoped for results:banghead:- namely decreased group size due to minimizing bullet smear. My gun has a .042 bullet jump at coal 1.275. 5.0 did show a 2 3/8" group, but that's nothing great as I've already printed a 1.375" group with 5.0 Red Dot at 1.275.
Many will say WHY BOTHER since it won't fit in the magazine anyway? If I can prove it does matter, I can make a custom chambered barrel with the right bullet jump to greatly improve accuracy.
Heck, it all part of the handloading accuracy game----right?