murph

[MarlinMurph458]

Good day
if my gun will handle close to the max OAL and function ok with the cast bullet I'm using, will there be any problems if the load calls for..... say 3.5 graines of a certain powder at oal length of 1.058 and I set the oal at 1.156? I know seating the bullet deeper than is called for can rise the pressure, but what about the other way around. Thank you
Murph

[rintinglen]

Pressure will drop if you increase OAL, taking velocities down with it, all other things being equal. Usually, the pressure drop is greater proportionally than the velocity drop.

In the 9mm, which is about the same size and length as what you are discussing, this is more pronounced than in a longer case, such as the 38 Special. The reason lies in rules of proportionality. A .1" increase in length as in your sample increases your combustion chamber by 9%. In the 38, which is half again as long, the combustion chamber is more than half again as long, so a .1 increase there will only have about 1/3 (actually less) of the effect. 6.0 grains of unique under a 125 JHP yields standard pressure, about 16,800 cup according to Lyman. 5.0 grains of Unique under a 125 grain jhp in the 9 mm yields about 30,700 cup, and travels about 150 fps faster. A 5.0 grain load in the 38 would only run about 700 FPs, versus the 1050 of the 9mm. The pressure though, would be only on the order 10,500 cup. So, yes, increasing OAL decreases pressure.

[spfd1903]

Practical example in 9mm, if I have slightly above minimum charge, and OAL is increased by say .02 to .03" the pistol will not cycle between rounds for a particular boolit weight.