Case OAL VS pressure?
- Thread starter Evil_Dark
- Start date
Effects of Seating Depth / COAL on Pressure and Velocity
The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.
http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-1/
At the opposite end of this is seating bullets into the rifling called "jam" at Accurate Shooter or Benchrest Central you will see postings with .020 off the lands or .010 into the lands.
With any reloads you start low and work up and when adjusting seating depth you also watch for signs of pressure increase.
When I fire form cases I jam the bullets into the rifling to hold the case against the bolt face and prevent case stretching.
The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.
http://www.bergerbullets.com/effects-of-cartridge-over-all-length-coal-and-cartridge-base-to-ogive-cbto-part-1/
At the opposite end of this is seating bullets into the rifling called "jam" at Accurate Shooter or Benchrest Central you will see postings with .020 off the lands or .010 into the lands.
With any reloads you start low and work up and when adjusting seating depth you also watch for signs of pressure increase.
When I fire form cases I jam the bullets into the rifling to hold the case against the bolt face and prevent case stretching.
No. If you are away from the lands then seating the bullet deeper in the case should increase pressure. However if you move the bullet out and it jams into the lands, that will increase pressure a bit too...
Jamming bullets into the lands is not a good idea. Basically it makes the round length at time of firing totally unpredictable and it also precludes the unfired round from being removed without firing. Seat your bullets .010 - .020" off the lands and don't worry about chamber pressure. As long as you follow the powder recipe in your manual you will be fine.
You need to read up on what competitive shooters are doing for accuracy and there is absolutly nothing wrong with "jamming" bullets into the rifling.
Now read up at Accurateshooter.com or at Benchrest Central.com.
Below are two links to accurateshooter.com about jam and experimenting with seating depth. And the only time the bullets will pull out of the case is if you do not have enough neck tension. (meaning your own fault)
Bugholes from Bipod
Precision Shooting with Froggy
http://www.accurateshooter.com/shooting-skills/bugholes-from-bipod/
"Now it’s time to seat the bullets. I use Redding Competition Micrometer bullet seaters. I set seating depth so that the bullet is jammed .005″ into the lands on my .308 and .010″ out of the lands on my 6.5-284. You have to experiment with what works in your rifle."
Forming 6BR Improved Cases
Three Methods for Moving Case Shoulder Forward
http://www.6mmbr.com/6improvedform.html
Method 1 -- Load to Jam and Shoot
The first, and simplest, method for forming 6BR Improved brass is to seat a bullet long into the lands, with strong neck tension, and then fire the case with a near full-power load.
Really? These targets were shot with the bullets jammed about 0.010" into the lands.
Yea, .010" is fine but they were talking about going .030" or more. Big difference.
I am well aware of the accuracy potential of what you are talking about.
AND, everyone bearing in mind that this is for target shooting, where you load rounds one at a time. Please do not use "jam" as a method of loading if the COL may then be too long for the magazine you expect to be able to load them in.
Unless you seat the bullet with a lot of neck tension, or crimp them, they are not going to stay at 0.030". My view is that once you go for a jam with a typical 0.002" neck tension you are just pushing it in as far as it wants to go. The theoretical jam is just a number... The real risk is that with certain combinations of jam and neck tension, the bullet will stay in the barrel if you change your mind about shooting and decide to extract the round. Not a good combination for hunting.
Correct. And my point with a large jam value is that when the bullet inevitably moves back there is no way to know how far and it would be silly to think each round will set back the exact same, which means the length just prior to firing will be totally inconsistent.
I get that. Small, controlled jam good. Big, uncontrolled jam not so much.
Well, My rifle is different I guess. I push my bullets 0.250" in the lands, as my chamber is soooo small, only 2.630", and I seat my bullets to have 2.820" or 2.880", depending on the bullet type. I remove unfired cartridges often from the chamber without any issues. If a measure back the cartridge, it's still ats its initial COAL, so no changes here. I have very good groups with multiples bullets weight and brands (sub MOA) so I don't have actually issues with that.
What I have noticed that is bizzare, it's when I tried to develop a load for 208gr ELD. The first ladder test I originally seated them by mistake at 2.820" COAL, then the second round of fine tunning the load I loaded them at 2.880", so all the velocities were different, much higher. See the results by yourself:
First ladder test at 2.820"
Then load tunning at 2.880"
Around 80fps faster or more! So that why I was confused about deeper = Slower...
Dark
What I have noticed that is bizzare, it's when I tried to develop a load for 208gr ELD. The first ladder test I originally seated them by mistake at 2.820" COAL, then the second round of fine tunning the load I loaded them at 2.880", so all the velocities were different, much higher. See the results by yourself:
First ladder test at 2.820"
Then load tunning at 2.880"
Around 80fps faster or more! So that why I was confused about deeper = Slower...
Dark
Well, My rifle is different I guess. I push my bullets 0.250" in the lands, as my chamber is soooo small, only 2.630", and I seat my bullets to have 2.820" or 2.880", depending on the bullet type. I remove unfired cartridges often from the chamber without any issues. If a measure back the cartridge, it's still ats its initial COAL, so no changes here. I have very good groups with multiples bullets weight and brands (sub MOA) so I don't have actually issues with that.
What I have noticed that is bizzare, it's when I tried to develop a load for 208gr ELD. The first ladder test I originally seated them by mistake at 2.820" COAL, then the second round of fine tunning the load I loaded them at 2.880", so all the velocities were different, much higher. See the results by yourself:
First ladder test at 2.820"
Then load tunning at 2.880"
Around 80fps faster or more! So that why I was confused about deeper = Slower...
Dark
You know that COAL has nothing consistent to do with a bullet's distance from the lands right? Tips or metaplats vary bullet to bullet, let alone between different types of bullets. You have to use a comparator to measure from the cartridge base to the spot where the bullet diameter is the same as the bore.
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Dark wrote "Around 80fps faster or more! So that why I was confused about deeper = Slower..."
It might be an English confusion issue. "Deeper" to me, means seating the bullet deeper in the case. OAL is shorter.
You might mean "deeper" means deeper to into the rifling.
My experience has been taking a familiar load and seating the bullets longer, so they hit the rifling. This has increased pressures/velocity.
Seating a rifle bullet deeper in the case does not seem to male any difference. It is more important in a pistol case..
It might be an English confusion issue. "Deeper" to me, means seating the bullet deeper in the case. OAL is shorter.
You might mean "deeper" means deeper to into the rifling.
My experience has been taking a familiar load and seating the bullets longer, so they hit the rifling. This has increased pressures/velocity.
Seating a rifle bullet deeper in the case does not seem to male any difference. It is more important in a pistol case..
Would it be any more variable than your seating die? Your not changing the headspace or throat demensions, why wouldn't they be different if pushed back off the ogive, What would change between each round?
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