Easy question, yes I did do a search first and didnt find the answer. What effect does seating depths as measured by COL have on pressure? My online search and in reading my load books are pretty inconsistent on data. Amount of grains of powder vary from source to source, so i always start low end and work up the ladder. COL varies from different bullet profiles also some hollow points have different profiles. So my question is seating too deep or too shallow does what to pressure? Does seating too shallow increase pressure? Thanks for taking the time to read my thread, and thanks to anyone who answers.
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Seating depths effect on pressure?
- Thread starter ragwd
- Start date
No definitive answer, but it could do either. Create too much space and you'll possibly create more pressure rather than less.
I don't worry about it because I build loads starting with the low end of the powder charge, and seat the bullet .020-.030 off the lands. I develop the powder charge with that length. Once I find my best powder charge, I "may" adjust the seating depth a bit. Moving the depth the minor amounts involved won't effect pressures enough to make any difference, so it is a moot point for me.
Sorry I didn't directly answer your concerns.
I don't worry about it because I build loads starting with the low end of the powder charge, and seat the bullet .020-.030 off the lands. I develop the powder charge with that length. Once I find my best powder charge, I "may" adjust the seating depth a bit. Moving the depth the minor amounts involved won't effect pressures enough to make any difference, so it is a moot point for me.
Sorry I didn't directly answer your concerns.
I would worry more about seating too deep - especially in handguns ....because I think you run the higher risk of getting higher pressures in that case...
Yes, OAL is all over the board...its the least valuable info in the loading tables in my view.../ I will often base my OAL on a similar bullet in a mfg's cartridge...and adjust as necessary.
I have several guns in each caliber of semi-auto handguns ( 9mm, .40 S&W, .45acp )...so I try and reach a compromise OAL that runs in everything I own ...from a 1911 with a 5" barrel to something small like a Sig 239...just in 9mm as an example ...
Yes, OAL is all over the board...its the least valuable info in the loading tables in my view.../ I will often base my OAL on a similar bullet in a mfg's cartridge...and adjust as necessary.
I have several guns in each caliber of semi-auto handguns ( 9mm, .40 S&W, .45acp )...so I try and reach a compromise OAL that runs in everything I own ...from a 1911 with a 5" barrel to something small like a Sig 239...just in 9mm as an example ...
The simple answer for hanguns is that a smaller case volume after bullet seating increases pressure and increasing that volume decreases volume. For the same bullet pressure increases as OAL decreases and vice versa.
For rifles it's less simple because a longer OAL gets close to the lands which then increases volume.
For rifles it's less simple because a longer OAL gets close to the lands which then increases volume.
For rifles it's less simple because a longer OAL gets close to the lands which then increases volume.
I think it was on this forum that someone posted a reasonable explanation of that. He described it as 2 separate pressure curves that you could plot on a chart. The first curve was caused by seating depth and decreased as the COAL got longer. The 2nd curve was the pressure spike caused as the bullet reached the lands.
Is this what you're looking for? See post #39 in this thread: http://thefiringline.com/forums/showthread.php?t=533900&highlight=bullet+lands+pressure&page=2.
That has a graph I've seen a few times.
That has a graph I've seen a few times.
What effect does seating depths as measured by COL have on pressure?
Rifle or Pistol? What cartridge?
So my question is seating too deep or too shallow does what to pressure? Does seating too shallow increase pressure? Thanks for taking the time to read my thread, and thanks to anyone who answers.
Actually not an easy question. There are many variables that go into an answer to that question. Type of powder and powder burn rates (stick, ball or flake), caliber you are reloading for, type of rifle it will be used in (semi-auto or bolt), temperature at which the round will be used in, type and brand of primer used, That is why it is always recommended that you start low and work your loads up for your rifle and components.
It is always more interesting to develop your own custom loads than just use commercial ammo.
Have fun and stay safe.
Jim
Thanks to everyone who took the time to post in this thread!! This was a issue for me when loading some of Berrys bullets of 9mm 124 grain THP. The profile while being considered a hollow point is a bit longer than other hollow points I have loaded. The col I get from my load books is 1.060 for hp, and with the HTP being a bit longer I loaded to 1.1 . So this is what brought the question of case volume and pressure. As for rifle I have loaded for my .223 and came up with a accurate bullet by going from 55 grain to a 69 grain , 1:7 twist barrel, and increasing powder a grain at a time till i found a sweet spot. Just a grain under Max. This was using Col from the book, so then i started increasing col by .015 at a time to try and get closer to the lands and it lost its accuracy. So thinking that the difference in case volume effected my pressure and lost accuracy, could I then adjust powder to get the accuracy back? would this take me over max? or just go back to the shorter Col that got me the original accuracy. would like to get closer to lands but not at the risk of going over max . Thanks for the help.
Jim Watson
New member
An old Vihtavuori pamphlet had some discussion of this.
By comparison with a factory rifle load, which is loaded to magazine length and probably with a substantial "jump" to the lands...
If you seat the bullet jammed into the lands or with very short "jump", pressure will be higher because of the engraving force.
If you seat the bullet very deeply, pressure will be higher because of reduced powder space.
In between, pressure will be lower due to ample powder space and free run to the lands. The factory load falls in this range.
So if you have best accuracy at book COL and a grain below maximum powder charge, you can do a lot of adjusting of both in a search for even better accuracy.
There is no particular virtue in loading close to the lands if it does not help accuracy.
Straight wall pistol cartridges are governed almost entirely by seating depth/powder space. "Almost" because you can get some effects from a short throat and a long bullet, as in some 9mms with heavy bullet subsonics in a short chamber throat.
By comparison with a factory rifle load, which is loaded to magazine length and probably with a substantial "jump" to the lands...
If you seat the bullet jammed into the lands or with very short "jump", pressure will be higher because of the engraving force.
If you seat the bullet very deeply, pressure will be higher because of reduced powder space.
In between, pressure will be lower due to ample powder space and free run to the lands. The factory load falls in this range.
So if you have best accuracy at book COL and a grain below maximum powder charge, you can do a lot of adjusting of both in a search for even better accuracy.
There is no particular virtue in loading close to the lands if it does not help accuracy.
Straight wall pistol cartridges are governed almost entirely by seating depth/powder space. "Almost" because you can get some effects from a short throat and a long bullet, as in some 9mms with heavy bullet subsonics in a short chamber throat.