Seating Depth, Velocity, & Pressures?

[kcofohio]

The cartridge is 9mm Luger. I have worked up a load from a published source to the max load. Which is near the max pressure of 35,000 PSI. But now I find the cartridge length fails the plunk test in a different gun.

I find the bullet needs seated .020" deeper for the second gun. So now, I'm looking to work up a load with all the same components and (1st) gun, with a shorter COAL and less powder.

My question is, with the work up of the shorter cartridge, can I expect to get the same velocity as the longer cartridge, and be at same pressure? Or, by reducing case volume, will the same pressure not produce a repeat of velocity?

Maybe I dozed off when this lesson was covered.

TIA

 

[Ironicaintit]

.020 is not a lot in the grand scheme of things.
You're playing it smart by dropping the powder too, then working it back up.
I think you will be able to get the velocity you had before without a problem.

 

[fiftybmg]

To prove to yourself one way or the other, chrony each round of different AOL in the same pistol.

I would not expect a meaningful velocity change by a 0.02 inch AOL reduction for 9mm, all other components remaining the same.

A change in velocity would be more noticable if one pistol had a longer barrel than the other.

 

[tightgroup tiger]

Here is the lesson you missed.

http://www.thehighroad.org/showthread.php?t=788813

You are on the right path by starting over but reading this thread will give you a good idea what the effects are in seating depths.

Unless your already at a compressed load seating to a shorter depth with less powder could keep the same pressure to a certain point.

With out knowing what powder and bullet your using it's hard to advise any further than this.

You said you loaded up until you failed the plunk test, explain, were you bulging cases with to much powder from compressed loads or OAL to long and bullet hitting the barrel leade?

 

[kcofohio]

Thanks for the replies!

tightgroup tiger, after doing the 1st work-up in one gun, I did the plunk test in the second gun, which failed the test. I didn't fire any out of the 2nd gun.

I used AA #5 & HS-6 in the 1st work-ups. I'm at work, so I don't have the data with me.

But as an example, (these are not true numbers) a max load: 124 gr. with 6.0 gr. HS-6 gave me 1,100 fps with a COAL of 1.075". And I need to shorten cartridge to 1.055", as I work up back to 1,100 fps and found that 5.8 gr. is what it takes.

My 1st load with 6.0 gr. @ 1.075" is listed as 34, 500 psi.
At 1.055" with 5.8 gr. and same velocity, can/should I expect the pressure to be at 34,500 psi.

Or is trying to attain the velocity while reducing casing volume, going to increase, or possibly spike the pressure.

 

[kcofohio]

A bit of background info. I have 3 handguns in 9mm. Primary test gun is not rated for +P. Of the other 2, one is rated for +P and the other (which failed the initial plunk test) is rated +P+.

tightgroup tiger, I did read the thread you linked, though on my phone. Informative results, to say the least. Maybe I missed a post that would have answered my OP.

I'll admit, I'm not the best with words. So if I left anything hazy, please allow me to expand.

 

[Walkalong]

I can't guess the velocity, but I would drop to 5.7 and start over.

 

[kcofohio]

Thanks for all the input!

I'll drop down .2 or .3 gr. and work back up.

I must have been over thinking this. I had noticed when looking for data, when data is listed for different bullets, say XTP and GDHP, with the same powder. The shorter of the two cartridges has a lower velocity with about the same pressure.

 

[Walkalong]

The depth of the bullet in case is more important than the overall length, plus the GD is made differently than the XTP. Speer often shows separate data for them vs standard jacketed bullets of the same weight.

Yep, over thinking like we all tend to do, especially in the beginning. Just adjust the OAL, drop the charge back, and work up again.

 

[kcofohio]

Just adjust the OAL, drop the charge
back, and work up again.

Thanks Walkalong, that is what I'll do.

 

[Potatohead]

Gamestalker often references his Speer #10 that mentions the earth will explode if you seat deeper. Im not so sure. I think it's overstated a bit. (no offense intended GS, and safety comes first, so its not a bad thing to perpetuate either way IMO)


+1 on overthinking. Im the worlds worst. Wish I would have had my "over thinking" problem back in school.


Whenever I get my farm I plan on doing a test with chrono, with different seating depths, to see how much the velocity is affected. My wife is deathly afraid of country living though, so dont hold your breath.

 

[kcofohio]

And where else can one go to get set straight, but THR!


Potatohead, some reason I had the theme song from Geen Acres playing in my head when you mentioned about a farm and you wife.

 

[Ironicaintit]

Gamestalker often references his Speer #10 that mentions the earth will explode if you seat deeper. Im not so sure. I think it's overstated a bit.

The loads in the speer #10 are hot! Maybe not all, but every caliber/powder ive referenced in it, had starting loads that were at least 1 grain of powder hotter than all the other MAX loads I could find.