I read on another forum that changing the COL of any centerfire round can cause a "DRASTIC" increase in pressure. It went on to say that a change in seating depth should be consisered a change in component and loads should be backed down and worked up again. I always thought that once I found an accurate and safe load I could just fine tune it a bit by changing the seating depth. Do I really have to back down and start over just cuz I changed the seating depth??


Thanks


.

The only time I've heard of COL causing pressure problems is if the bullet is jammed into the lands, in which case pressure can rise quite dramatically.

I'm a bit sleep deprived, but as I recall John Barness had an article in handloader in which pressure readings were taken with changes in COL, and the pressure change wasn't signifigant until the bullet was seated into the lands.

I also start loadwork by varying powder charges, and use seating depth to fine tune the load.

The threshold of extractor groove growth [egg] is the highest load when the bullet almost touches the lands. Shorter seating or longer seating drive up the pressure and make the threshold load come down in number of grains.


.223 35 gr. .224" Vmax moly, Blue Dot:

17 gr. 2.045" .001" egg
17 gr. 2.138" no egg
17 gr. 2.170, .029" egg, primer fell out


The correct answer is 15 gr 2.170", 3,500 fps

Thanks Paul, They were not talking about seating to the lands. They stated that the change in case volume due to a seating depth change would cause a "DRASTIC" increase in pressure. No mention was made to an increase in pressure due to the bullet being pig-jammed into the lands. The word "DRASTIC" was emphasized over and over.

steve4102 Quote – “I read on another forum that changing the COL of any centerfire round can cause a "DRASTIC" increase in pressure. It went on to say that a change in seating depth should be consisered a change in component and loads should be backed down and worked up again.â€

I believe reducing case capacity will increase pressure. I don’t believe the difference will always be drastic, but in some instances it might. I believe if you are at a MAX pressure powder charge and increase the OAL the pressure will be reduced (as long as your off the lands), but if your at a MAX pressure powder charge (off the lands) and reduce OAL pressures will increase above MAX. I’m not sure but maybe smaller cases are more prone to bigger pressure differences when reducing OAL, 9mm comes to mind.

quote:

I believe reducing case capacity will increase pressure.

Actually, it's just the opposite, according to the Hornady manual, which has quite a discussion about it.

Three cases:
(1) Normal, nominally 0.030 JTL (Jump To Lands), safe pressure and 3500 fps (in their example);
(2) Seat more deeply, much lower pressure and 3400 fps (again their example) for the same load, due to greater expansion volume after the powder has started expanding. (The freebore serves as expansion volume, and there's more of it when the bullet is seated more deeply.)
(3) Touching the lands, velocity is (perhaps) dangerously high to achieve the 3650 fps, as the bullet did not get a running start before being slowed by the rifling.

Jaywalker

Jaywalker

I sure wouldn’t want to reduce seating depth in handguns and expect less pressure.

In rifles I guess the amount of freebore makes a difference. Here is a test I did.

I’m shooting 308 in a Shilen barrel 1/12 twist. I am shooting Burger Moly 185gr VLDs. I used Lapua brass that has been fire formed. The two cartridges were identical except one was seated into the lands and the other was seated off the lands.

The Jam length for my rifle measured 3.300. The cartridge that had the bullet seated into the lands measured 3.297. The cartridge that had the bullet seated off the lands measured 3.267. The distance to the start of the lands measured 3.277. All measurements were made using a Sinclair bullet comparator.

One cartridge had the bullet seated into the lands .020. I loaded 3 of these cartridges.
One cartridge had the bullet seated off the lands .010. I loaded 3 of these cartridges.
All were shot through my chronograph and velocities are listed in fps.
N550 powder 44.0gr was used for all loads.

Bullet seated into the lands .020 –
Average - 2488
High – 2500
Low – 2480
ES – 20.1
SD – 10.5
AD – 7.9

Bullet seated off the lands .010 –
Average - 2508
High – 2517
Low – 2501
ES – 15.7
SD – 8.0
AD – 5.8


More Velocity seated off the lands. Go figure?

quote:

Originally posted by steve4102:
I read on another forum that changing the COL of any centerfire round can cause a "DRASTIC" increase in pressure. It went on to say that a change in seating depth should be consisered a change in component and loads should be backed down and worked up again. I always thought that once I found an accurate and safe load I could just fine tune it a bit by changing the seating depth. Do I really have to back down and start over just cuz I changed the seating depth??

Hey Steve, Sounds like a good Forum to avoid, because it has created a Logic problem for you.

First off, you are getting some good answers here at AR. However, some of it appears to be in conflict because they are talking about totally different "individual cartridges" without making a comparison for you.

It will help you to think of this situation in a Volumetric Relationship as well as an Altered Burning Rate.

Let's get the Volume issue on track, since it will help you get ahold of the confusion factor.

quote:

Originally posted by Bullet94:
I believe reducing case capacity will increase pressure. I don’t believe the difference will always be drastic, but in some instances it might. ...I’m not sure but maybe smaller cases are more prone to bigger pressure differences when reducing OAL, 9mm comes to mind..

Here Bullet94 guessed correctly. His example of the 9mm is in fact one that can create a Drastic Pressure increase by Seating Deeper because it affects a Higher PerCentage of the Case Volume when compared to say a 357Mag. So if you Seat Deeper in both cartridges, you would normally see more Pressure Increase in the 9mm.

When you get to rifle cartridges, similar relationships apply. If you were to compare the Pressure change between a 264WinMag and a 260Rem by Seating Deeper(the same distance deeper), you will typically see more Pressure Increase in the 260Rem.

If you do the same thing with a 260Rem and a 6.5TCU(6.5x223Rem), then the 6.5TCU will typically increase the Pressure more than in the 260Rem.

In all the above situations, it is easy to understand if you look at the Volumetric Relationship. The smaller the Internal Case Volume, the more effect will be noticed because of the Higher PerCentage of Case Volume Reduction.
---

Then you get to the Altering Burn Rate.

When the Bullet is Kissing-the-Lands or Seated Into-the-Lands, the Pressure Curve of the burning Powder is Altered so the Pressure Spike is Higher and occurs Faster than when the Bullet is Seated Off-the-Lands(Seated Deeper).

This is also simple to understand if you think of the Bullet being "chocked in place"(like putting a brick in front of a truck tire). It will take you more effort - Higher Pressure - to get the Truck rolling with the chock in place because you have to not only over-come the Static Inertia of the Truck, but you also have the additional Force needed to over-come the chock.

In the Bullet situation, it is quite similar and it Alters the Burn Rate. That "has the potential" to create a Drastic Increase, but it does not happen in every situation. It depends on the specific Case, Primer, Powder and Bullet used in a specific Rifle.

Best of luck to you(especially if you continue to hang at that "looser" Forum ).

My own testing has shown that a longer COL results in higher velocity.

The other difference with small pistol cartridges versus larger rifle cartridges is the powder burning rate - the pistol cartridges are loaded with much faster burning powders. In normal loads peak pressures are reached much sooner in pistol cartridges than rifle cartridges, so all of the bad effects already listed in Hot Cores reply happen much faster in a small pistol case loaded with a really fast powder like Bullseye than they would in a large rifle case loaded with 4831 or 4895.

Hot Core
Excellent explanations. I also enjoyed your brick under the truck tire anology.

muck
PS. Wasn't the long freebore Roy Weatherby's trick to getting "extra" velocity for his rifles?

quote:

Originally posted by muck:
... Wasn't the long freebore Roy Weatherby's trick to getting "extra" velocity for his rifles?

Hey Muck, Thank you. Sometimes it is difficult to come up with an example that people can relate to.
---

I agree that increasing the Freebore was one of the things Mr. Weatherby did to achieve higher Velocities - Safely.

It causes the Peak Pressure Spike to occur slower in the firing sequence and at a reduced amplitude. Even though the "Peak" Pressure is reduced, the Average Pressure can be higher for a longer duration, thus more Force is applied to the Bullet Base which increases the Velocity.

We all benefit from the visionaries like Mr. Weatherby.

Jaywalker Quote –
“quote: I believe reducing case capacity will increase pressure.

Actually, it's just the opposite, according to the Hornady manual, which has quite a discussion about it.

Three cases:
(1) Normal, nominally 0.030 JTL (Jump To Lands), safe pressure and 3500 fps (in their example);
(2) Seat more deeply, much lower pressure and 3400 fps (again their example) for the same load, due to greater expansion volume after the powder has started expanding. (The freebore serves as expansion volume, and there's more of it when the bullet is seated more deeply.)
(3) Touching the lands, velocity is (perhaps) dangerously high to achieve the 3650 fps, as the bullet did not get a running start before being slowed by the rifling.â€



Jaywalker your post had me confused until someone on another forum pointed this out -
When Hornady said, “Seat more deeplyâ€, they meant closer to the lands, not deeper into the case.

Because Pistol cases are smaller a change in seating depth will have a proportionally greater impact on pressure. Too deep and the pressure will go up and too high and you may hit the lands… I always approach with caution any change… Rifles seem to be more forgiving… given you keep off the lands

It is important to understand that the changes in pressures and are usually not linear. In other words, a 5% reduction in case volume due to a bullet seated too deep will not necessarily result in a 5% increase in pressure. Most often (I would say always, but sure enough somewhere out there is the one case that woul dmake a liar out of me!) it is more. At the pressures go way up, as in modern high pressure loads, the change is greater. 5% in 30-30 or 45ACP would have less effect than 5% in a 30-06 or 9mm, for examples.

At some point, it begins to be a runaway where very small changes make very large effects. The 357Sig cartridge is often criticized for this, as it is high pressure (strike one), low volume (strike two) and had a very short neck (strike three) which can allow bullets to get set back more easily...though any of the high pressure small volume pistol cartridges are vulnerable.

Also, keep in mind that a .01" setback of the bullet is VERY small (percentage of case volume wise) in a 30-06, but quite significant in a 9mm/40S&W/357Sig type cartridge. It all adds up, and sometimes it multiplies up!

Hi All,

This is my first post on this site, but I have been looking at the site for quite sometime. I wanted to post earlier on this topic, but was having a problem with getting on. I just wanted to say that in Steve's original post he mentioned that the advice was, since the changing of the COL was considered a component change, that working up the load was recommended. While the answers given, did seem to create a lot of different opinions which is often true, I did not see where that opinion was addressed. So, I just wanted to say that from a safety stand point, the original advice was very valid. Anytime a component is changed, it is a good practice to reduce the load and work it up again. I practice this measure religiously.