OAL gauges, modified cases, seating depths for accuracy etc

[Timmis2]

I've read that reloaders will use a OAL gauge, modified case, bullet comparator, and projectile (or other means) to measure distance to the lands to determine their seating depths for accuracy tests. The recommended OAL/COL starting point is .002" less than where the bullet touches the lands (or the portion of the chamber/bore the projectile is obstructed). When reloaders use this COL to begin accuracy tests, will they usually keep the same .002" COL and adjust powder charges to test for accuracy? Or will they keep the same powder charge and reduce the COL incrementally to look for differences in accuracy?

I have read that pressure increases as seating depth is extended closer to the lands. I assume this is because less gas can escape out the barrel before the bullet obturates and seals the chamber. But it is also said that pressure decreases as your seating depth is set longer. Is the "pressure decrease from longer seating depth" theory usually just a measure of pressure difference inside the case? If it is a measure of overall pressure difference in the chamber, then that contradicts the theory that pressure increases as bullets are seated closer to the lands. Perhaps most reloaders are seating their bullets far enough from the lands that there is usually an overall decrease in pressure with longer OAL's, and as your OAL is set closer and closer to the lands, the pressure difference may shift from a decrease to an increase.

Also, isn't it true that when using a fast burning pistol powder in a rifle cartridge with reduced powder charges (for example Hodgdon publishes 3.6gr of Titegroup for subsonic .223 loads), pressures increase and may spike dangerously if COL's are set longer due to the possible reaction of pistol powder detonation inside a case with that much empty case space? This contradicts "pressure decrease from longer seating depths" too, but I imagine there's a whole different set of rules and reactions when dealing with fast powders and empty case space.

 

[K22]

The answers to some of your questions depends on your shooting discipline and accuracy requirements. I can only share my process where my goal is varmint grade accuracy (about .5 moa or so)

Point 1: I think you mean .020" from lands. At least that is my starting point assuming at least one bullet diameter (minus the Boat Tail portion) is inserted to the case to provide adequate bullet tension (grip on the bullet) and the cartridge will fit the magazine. I never seat closer than .010" from the lands due to variations in bullet ogives because I do not want the bullet either touching or jammed into the lands.

Point 2: When I conduct load development, I test various powder charges keeping COL constant for a given bullet. I only change one variable at a time. In my experience, bullet selection along with the powder chosen and resultant charges are the most significant factors affecting accuracy. Seating depth can make a significant difference in some rifles. I have at least one where this is the case. Whereas I have others where, within reason, there is little to no measurable effect on target. Once I find a bullet / powder / charge combo that meets my accuracy requirements or close to it, I may try some different seating depths if necessary.

Point 3: Pressure - stick to published data from reliable and lab tested sources such as Speer, Lyman, Sierra, Hornady, etc. Jamming bullets into the lands can increase pressure. I am not a fan of doing that. I never had trouble finding a serviceable load without resorting to jamming the bullet. Most manuals provide the COL that the load was tested at. If you adhere to that with some reasonable variation for testing for accuracy you should be fine as long as you do not jam the bullet into the lands.

Point 4: Use reliable published data for reduced loads. The issue of denotation due to reduced charges was tested by White Laboratories many years ago and it was never conclusively proven but my information on this issue is quite dated. There may be new testing that sheds light of this issue.

I consider safety the most important element in reloading. In almost 60 years of reloading, I never found it necessary to deviate from published data or the safety guidelines stated in reloading manuals. Keep in mind that if you change components from those listed in published data, pressure can change. So be conservative in your powder charge selection. Learn how to read pressure signs. At the first sign of high pressure, stop, reassess your load.

 

[JMayo]

In rifle It's base to ogive - BTO not col. The tip doesn't matter except in magazines it's gotta fit.

Most i know do "the wheeler method" . Take FP & ejector off the bolt, to get shoulder and touch established.
Start at touch and go.002 at a time which ever way you want to go.
Allot of r&r had been done shorting the learning curve, imo

 

[charlie b]

Seating depth vs pressure.

1. As the bullet is seated further out the case volume increases and the peak pressure will decrease.
2. As the bullet gets close to or touches the lands the amount of force to push the bullet into the lands will case a pressure increase.
3. Yes, 1 and 2 are contradictory.
Most bullets work well by 'jumping' the bullet, ie, seating short of the lands, usually by .020" or more. Start with the OAL indicated by the bullet mfg if you can find data from them. Until you get a bit of experience in reloading and shooting just use the mfg data for seating depth.

Using small loads, especially pistol powders, in rifle cartridges is a different game. The cast bullet folks have embraced this area of reloading and you can find a wealth of information from them. There are many cautions when using reduced loads. There are also a lot of 'old wives tales' about reduced loads. If you are interested in this area get a copy of the Lyman Cast Bullet Handbook (No3 can be downloaded for free). It also has a ton of information on reloading in general. Lyman's reloading handbook is also a good reference for general reloading. Then go to the Cast Boolit forum and see how others do it.

 

[Straightshooter1]

Seating depth vs pressure.

1. As the bullet is seated further out the case volume increases and the peak pressure will decrease.
2. As the bullet gets close to or touches the lands the amount of force to push the bullet into the lands will case a pressure increase.
3. Yes, 1 and 2 are contradictory.
Most bullets work well by 'jumping' the bullet, ie, seating short of the lands, usually by .020" or more. Start with the OAL indicated by the bullet mfg if you can find data from them. Until you get a bit of experience in reloading and shooting just use the mfg data for seating depth.

Using small loads, especially pistol powders, in rifle cartridges is a different game. The cast bullet folks have embraced this area of reloading and you can find a wealth of information from them. There are many cautions when using reduced loads. There are also a lot of 'old wives tales' about reduced loads. If you are interested in this area get a copy of the Lyman Cast Bullet Handbook (No3 can be downloaded for free). It also has a ton of information on reloading in general. Lyman's reloading handbook is also a good reference for general reloading. Then go to the Cast Boolit forum and see how others do it.

When I looked at my data, I was puzzled why my velocity would decrease as I seated further in during my seating depth load developments. A few years back I finally found an old study that addressed that and I found it fascinating that the blowby had such a significant effect. Here a graph they had that gives a good visual about that:

 

[JFrank]

Just to stir the pot s little .
I ran a seating test today, jamming the bullets incrementally from .006 - .014 past touch.
The beginning shots were nearly identical in speed as the ending shots while the middle two shots ran a bit faster. Those I’ll attribute to something beyond simple seating causing chamber pressure variations.
Jim

 

[Bill Norris]

Just a point to remember. Sometimes a hand loader will decide mid-stream that they need to test with bullet jam. If and when this is the case the powder charge MUST be reduced and load re-worked for safety purposes.

 

[RegionRat]

FWIW: Here is some raw data for a 6 Dasher with a 105.

The bullet is into the lands is on the left, and more jump is to the right, so the case volume goes up to the left and down to the right. The charge is fixed.

 

[JFrank]

Just to stir the pot a bit further and of course let’s all adhere to safety first and always stop at any pressure signs.
A few notes from a recent seating test on my BRA going from .009 jam to a .024 jam, I note s 10 ~ fps change from beginning to end.

 

[Timmis2]

For those of you who tested this and found a predictable loss in velocity and pressure with more of a bullet jump, did you notice any gains or losses in accuracy?

Also, If you want to adjust your seating depths to test for accuracy, would you typically avoid bullets with cannelures since bullets with cannelures seem to be made to be seated at a set depth?

 

[Straightshooter1]

For those of you who tested this and found a predictable loss in velocity and pressure with more of a bullet jump, did you notice any gains or losses in accuracy?

No real change in accuracy, but there were changes in precision.

 

[Rat Rifle]

FWIW: Here is some raw data for a 6 Dasher with a 105.

The bullet is into the lands is on the left, and more jump is to the right, so the case volume goes up to the left and down to the right. The charge is fixed.


View attachment 1689413

I suspect, supported by post #5, if this test were continued, at some point that curve would start going back up ????

 

[dellet]

The reason for all the drama and seeming contradictions over changing seating depth and what happens to pressure and velocity is that it violates the most basic fundamentals of loading. Only change one thing at a time. You can’t move a bullet in or out of a case without changing both internal case capacity/load density, without also changing chambers volume. If you were to overlay graphs of pressure and velocity changes while increasing/decreasing chamber volume with increasing/decreasing load density, you would get an X pattern. A point where one change has more or less influence than the other. Think of the importance of case volume using Quickload GRT.

The only thing for certain is that if you look at enough studies, you will find contradictions and determine that everyone is right in their own little world.

Someone who is limited to magazine length and is adjusting overall length is more likely to see velocity and pressure increase with a deeper seated depth than someone who can jump and jam their bullet. But if you load from the magazine and use a bullet that jams at less than magazine length, it changes the test or study.

Case capacity or load density largely determines how fast the powder ignites. That more or less determines starting pressure and whether the bullet slides out of the case mouth, or the neck opens to release the bullet. Different powders, different answers. Add a crimp, change the answer. Change the jump, change the answer because it’s easier to push the bullet than expand the case neck, but when the bullet hits the lands, it’s no different than the bullet hitting an obstruction in the barrel. The difference is timing of the pressure spike and force needed to overcome the obstruction,

Remember that smokeless powder needs pressure to burn, the more pressure, the faster it burns, the faster it burns, the more pressure it creates. One of the ways to create more or less pressure is adjust case capacity. If case capacity, therefore load density doesn’t matter, why is it probably responsible for the most common and gross error using Quickload or GRT?

Extreme example of case capacity, bullet weight and bearing surface friction test I’ve done was 300 Blackout subsonic. 11.5 grains of 4227 pushes both a 125 and 240 grain bullet the same velocity.

Another Blackout story and assuming published data is tested and safe. Hornady 9th edition came out about the time 300 Blackout hit the market. It had the most extensive data at the time. The 150 FMJ bullet was one of the most popular cheap bullets to use, almost every manufacturer has one. People that used Hornady data found they could use any 150 FMJ bullet out there, except the Hornady. If you used the Hornady bullet at magazine length, their suggested and published COL, you had flattened or popped primers at minimum charge weights. The problem was that the Hornady 150FMJ, was the only one that is a hard jam at 2.260”. Some powders reacted to this much different than others. The Lilgun experience was much more exciting than 1680. more so if you jumped straight to max load.

For every hard truth, there will be an opposing hard truth, simply because you are changing two things at once when adjusting seated depth. Load density and chamber volume.

 

[Bill Norris]

dellet said​

For every hard truth, there will be an opposing hard truth, simply because you are changing two things at once when adjusting seated depth. Load density and chamber volume.

And this is the reason why bullet quality should be included in the title of the thread. If changing seat depth a couple of thousandths matters then the position of the ogive on the profile of the bullet matters as well. Even though the ogive measurements are taken with good tools and practices there will be a difference in the length of bullet in the case if the ogive is at random on the profile.

 

[JFrank]

Keep in mind that I’m only moving a bullet less than .020 into the lands on approx 80-90 % compressed load.
A relatively small amount compared to the graph in post #8 of .080 jump while further compressing the load yet the graph shows a decrease in velocity of 33 ~ fps.. assume blow by is a factor.
It’s always interesting to compare computor generated charts and graphs to personal recorded data and how these affect the target.

 

[RegionRat]

I suspect, supported by post #5, if this test were continued, at some point that curve would start going back up ????

Yes. However.....

When we remember to state the context, we add clarity to the forum....

A BR match is run differently that a PRS, Highpower, or F-Class match, so the goals and methods are related but when it comes to putting a fine point on state of the art performance, I am not talking about a BR gun. If I make comments here, they are oriented differently to what I would do in a BR match.

My graph's context was that the 6mm Berger 105 Hybrid in a Bartein bbl is a known entity. It has been run by so many that we know what it tends to do.

Same for the Alpha Brass, Varget, and the CCI 450, but the point of this comment is that when the idea is that a batch of pre-loaded ammo is being developed for a given barrel, we still end up exploring the "personality" of that chamber and barrel.

Fine seating tests and charge tests do show some amazing performance that will only last as long as conditions stay similar. In my world, we shoot 20 shot strings X3 plus sighters for the day, but rarely does that ever happen fast enough to keep that sort of tuning from the first string to the last if the sun comes out.

I wouldn't be very interested in going into two issues, which are compression loads or having the boat tail junction going down into the case. I also don't jam for that sort of match, but this barrel shoots bugholes when using jam.

So for the OP's sake, the point of mentioning this, is we can tune on the spot for small advantages that might be found with a given recipe but are dependent on the current conditions of the barrel and the weather.

Very fine seating tests or charge tests tend to show promise, but if you load a box of say 100 rounds based on your results on a given day, and then wait a week and shoot them at a different venue in different weather, do not expect to see the identical results.

A good recipe in a good barrel is not going to head south on a load like that, but it may not shoot into the ones or twos. As long as it shoots a ragged three or four, I know I got something for the trouble.

I could give that barrel anything within reason and it will shoot roughly 0.4 to 0.5 MOA with that bullet and brass up to 600 yards. It does take work to keep it in the twos and threes, but that can shift with weather and conditions. I also have to do the homework if I send those to 1000 yards and expect the gun to keep them under 5 inches.

So to a degree we make our own luck by using the right equipment and materials. In loose terms two thirds of the battle is won by picking the right stuff. The last third of the gun's performance is on you in terms of ammo workmanship and selecting the charge and depth.

Pick a known good platform, and pick a known good recipe, then get to work and you will learn this in time. It takes a lot of shots to learn to get things working at 300, then again and 600, and another learning curve to get that to 1000 or more. You can skip a lot of pain by starting with a good platform and recipe. Good Luck, YMMV

 

[RegionRat]

For those of you who tested this and found a predictable loss in velocity and pressure with more of a bullet jump, did you notice any gains or losses in accuracy?

Yes and no, but as has been stated above, the average POI at say 300 or 600 yards is one thing, the group size is another.

If I jam my load with a Berger 105 Hybrid in a good bbl, I can get some amazing groups. But will that last 60 to 80 shots at a fixed depth? Answer is no, so I still search for a more stable jump value that tends to hold for hundreds of shots while the bbl wears out.

BTW, I should point out the the Berger 105 Hybrid has a shape that is known to be very forgiving to jump while performing very well. The main point of mentioning this, is if you pick a bullet known for being easy to develop (tangent ogive), versus one that is picky (secant VLD ogive), you can change your outcome.

Also, If you want to adjust your seating depths to test for accuracy, would you typically avoid bullets with cannelures since bullets with cannelures seem to be made to be seated at a set depth?

In general, yes to avoid.
Unless we are forced to discuss some LEO/MIL or hunting topics, the general discussion of doing fine seating depth searches is not typical for cannelure bullets.

It goes without saying we are not loading for machine guns in this discussion, or ones that require a cannelure or crimp to prevent set-back.

You can always load a cannelure bullet at some other depth, but in general if you are trying to learn precision and accuracy, make it easy on yourself and stick to known good platforms with recipes that are known to be easy to develop, then go learn to shoot.