Lee Loader

Hi Guy's

Still trying to learn, I've been watching Videos about the Plunk Test. What is the significance of this,
if your load data tells you your OAL?
Another question is about Pressure, the deeper you set the bullet, the more pressure you get and vice versa. Right?
So, If someone is loading at say, OAL 1.085 and I'm loading the exact same recipe at OAL 1.060, What is the outcome of both loads?

Is Plunk Testing only to determine Max OAL?

Plunk test is usually used to check sizing and seating depth to ensure that the cartridge "plunks" in the chamber . Some chambers are cut in such a way that seating a particular bullet is critical to chambering . Start with Magazine length and move to chamber length .

Changes in OAL of .02 are huge in pistols where the difference between a Special and a Mag or the next cartridge up are .1 say .380 to 9mm Schofield to Colts to 454 . In 38 Special that can be 2-3,000 psi with a WC 148 . It's a lesser consideration in say a 45-70 . If loads are start to below half of the load window in the 50kpsi + rifle cartridges it's a lesser issue but with an extreme movement may cause increases beyond limits if you're shortening below test lengths .

Data is generated within a set of standards so that every set is set at the same standards . Every deviation from those standards makes a change .
124 gr 9/38/357 bullets for example will be shorter as a WC than a long ogive HP , even a standard HP will be longer than a hemispheric RN . Each set at the same OAL will have significant differences in case intrusion which is where pressures come into play .

Start low work up . If you change something work it up again .

RBHarter wrote:Plunk test is usually used to check sizing and seating depth to ensure that the cartridge "plunks" in the chamber . Some chambers are cut in such a way that seating a particular bullet is critical to chambering . Start with Magazine length and move to chamber length .

Changes in OAL of .02 are huge in pistols where the difference between a Special and a Mag or the next cartridge up are .1 say .380 to 9mm Schofield to Colts to 454 . In 38 Special that can be 2-3,000 psi with a WC 148 .

Start low work up . If you change something work it up again .

My thoughts exactly, especially the last line...

larryw wrote:

RBHarter wrote:Plunk test is usually used to check sizing and seating depth to ensure that the cartridge "plunks" in the chamber . Some chambers are cut in such a way that seating a particular bullet is critical to chambering . Start with Magazine length and move to chamber length .

Changes in OAL of .02 are huge in pistols where the difference between a Special and a Mag or the next cartridge up are .1 say .380 to 9mm Schofield to Colts to 454 . In 38 Special that can be 2-3,000 psi with a WC 148 .

Start low work up . If you change something work it up again .

My thoughts exactly, especially the last line...

So When you say changes in OAL of 0.2 does that mean 1.075 (minus .02) = 1.055, I just want to make sure that's what you mean. Thank you!

Yes , that's correct .

The smaller the case , the bigger the difference for changes in length .

RBHarter wrote:Yes , that's correct .

The smaller the case, the bigger the difference for changes in length.

So, if the seating fluctuation is off by -0.2 (1.055) from 1.075 that I'm going for, will that change the pressure build-up in the cartridge and shoot hotter?

Does the space between the Powder and the Bottom of the bullet matter inside the finished cartridge?

Just wanting to understand all this, Thank you!

Good afternoon
Yep. Shorten the combustion chamber (inside the brass) and pressure has to rise all other factors equal.
High compression engines can have taller pistons to accomplish the pressure increase. Less space for the expanding gas.
Same within a loaded cartridge although for a short time

As Missionary mentioned, a cartridge is just a combustion chamber, the bullet being the "piston" (If one is familiar with internal combustion engines). Smaller combustion chamber with same amount of fuel will create higher chamber pressure, pushing the piston (bullet) faster. Engines can have the heads milled and shaped pistons to decrease the combustion chamber, raising pressure and bullets seated deeper raise chamber pressure.

As an example I ran some numbers through Quickload. Powder chosen was Bullseye. Starting load and O.A.L taken from Lyman 49th Edition. I chose the calibres as the bullets are close in diameter and both calibres are popular.

9mm Luger - 115 Hornady HP/XTP - Starting O.A.L. 1.090 - 3.5 Grns Charge
A change to 1.070 O.A.L. caused a 6.5 % decrease in free space in case and a 8.5% increase in calculated pressure.

357 Magnum - 158 Hornady JHP - Starting O.A.L. 1.590 - 6.5 Grns Charge
A change to 1.570 O.A.L. caused a 3.4% decrease in free space in case and a 6.0% increase in calculated pressure.

This example is illustrative only as the results are calculated not measured. Seating depth is more critical for the smaller 9mm case versus the larger 357 Magnum as has been said in other replies above and illustrated in this example.

Hope this helps.