Seating depth with new and once fired brass

[CanMike]

I think I get the math but wanted to make sure before I seat some bullets.

I found seating depth on virgin lapua brass as best at 0.010 off lands. Let’s say CBTO of 2.200

Once fired brass shoulder to base grows 0.006

So in order to get same seating depth I need to load the once fired brass 0.006 longer CBTO to 2.206

Am I understanding it correctly?

Mike

 

[spife7980]

Am I understanding it correctly?

No, well maybe. Depends on how you arrived at your initial measurement. The bolt face to the point that the bullet touches the lands hasnt changed.
If your measurement was to the case head instead of the bolt face such as with the hornady tool, then yes, adjust accordingly.
If you do the wheeler method that pushes the bras up against the bolt face, then no, dont change.

 

[RegionRat]

If there is a 0.006" difference between your fired brass and your virgin brass, the answer depends on several other variables and that means this is another "it depends" answer....

You might shoot that virgin recipe and tell us you can detect some "significant" improvement based on your 2.200" recipe.
The question would be, how wide was the window to begin with when using virgin brass? Was it sensitive to less than 0.006"?

Let's say you are now shooting fired brass and you minimally bump it. Will the change in the brass cause the difference or will the shoulder distance to the lands cause the difference?

You can tell where this is headed, which is to say that only a firing test will tell since you are also making changes to the brass.

Many guns and shooters cannot detect the difference if they leave the CBTO value the same when you consider the difference between their virgin and fired brass, while others can demonstrate it. YMMV

 

[CanMike]

Thanks for the answers.

How wide the node is I don’t know as only did preliminary seating with the virgin brass. Finalizing with fire formed.

10 was better then 15 off which was better then 20 and 25 So I am guessing my window is 5-12 or so as groups started tightening at 15 versus bigger jump.

I measure lands with both Hornady tool and case with bullet / close bolt. Both showed similar numbers. The Hornady modified case is almost identical dimensions to the virgin lapua brass.

Mike

 

[CanMike]

Also should add I am not set on a specific jump as that’s kinda arbitrary and not 100% accurate with measurement tools I have. I just want to match seating depth between the virgin and one fired. Basically want to replicate distance with the once fired brass as I have with virgin.

Mike

 

[Sig Marine]

Bullet distance to the lands is one thing. Case volume is another.

 

[CanMike]

Yup. I’m going to be adjusting powder charge a few tenths. Thats why I wanted to keep jump same and the. Adjust powder to get back to same velocity. Figure probably .2-.3 less powder. But will play with that.

Mike

 

[Sig Marine]

CBTO measurement changes pressure as does the powder charge. I look at powder as a “course” tune and seating depth as a “fine” tune. Finding each window offers some flexibility with a particular load for a specific barrel and bullet combination.

 

[straightshooter1]

I think I get the math but wanted to make sure before I seat some bullets.

I found seating depth on virgin lapua brass as best at 0.010 off lands. Let’s say CBTO of 2.200

Once fired brass shoulder to base grows 0.006

So in order to get same seating depth I need to load the once fired brass 0.006 longer CBTO to 2.206

Am I understanding it correctly?

Mike

There's a few things at play with your scenario. .006" expansion is quite a lot, though not particularly unusual. That does have some effect on how the case volume effects ignition, and if you increase the seating depth by that amount, that'll certainly change the volume the powder works within enough make a difference. Keeping the same seating depth would mitigate what little effect there is for the change in volume. The big issue I see here is that starting at .010" off the lands and brining it .006" closer is going to increase your start pressure by a lot and that alone will effect a change in your results significantly. If you had started further back from the lands (like .020 or .030 off), adding .006" wouldn't have as big of an effect. Just don't expect adding .006" to give you the same results.

 

[CanMike]

Okay maybe I am missing something or not explaining properly.. My goal is to stay 10 of the lands with the new 0.006 larger case.. I assumed (and I can be wrong) that the 0.006 expansion is neck down so the case is longer from base to neck by 0.006 so when seating a bullet I need to seat 0.006 longer to maintain that 10 off the lands distance.

I can be completely off.

And yes 100% agree with pressure, etc if I were to bring in 4 tenths off the lands. (Which is what I am trying to avoid)

Mike

 

[Bullseye Smith]

I think I get the math but wanted to make sure before I seat some bullets.

I found seating depth on virgin lapua brass as best at 0.010 off lands. Let’s say CBTO of 2.200

Once fired brass shoulder to base grows 0.006

So in order to get same seating depth I need to load the once fired brass 0.006 longer CBTO to 2.206

Am I understanding it correctly?

Mike

Wouldn't it be right to keep the die set the same for new brass and that would be the bullet is only set .006 deeper into the brass. Then the overall measurement would be 2.200 which is .010 off the lands! Then trim after two loads or three.

 

[RegionRat]

How close you will stay to your original jump dimension when changing the brass prep, is dependent on the method you used to establish it in the first place.

Some folks think they have the "touch point" located with their Stoney Point tools (now Hornady) and some folks use a dummy cartridge with a stripped bolt. There are probably at least a half dozen variations on how to get a "touch point".

Between those two methods, there is a difference in the use of the shoulder as a reference datum, versus the case base and the bolt face as the datum. So there is no point to debating the use of the case shoulder versus the case base, because there is often as much uncertainty in how folks jam that bullet.

You could have what we call a round robin session where we hand the rig and tools to several "competent" folks, and get a difference of nearly 0.006" just based on how folks define the "touch" point. The value is force and friction dependent but that isn't the end of the world.

What I will recommend to you is this. Pick your favorite method and create a dedicated dummy round and call that a reference. Then, stop worrying it and just run your target testing and track the ammo seating depth against that reference round. The fact that your brass volume will change, the shoulder datum length will change, and that the throat will wear, are unavoidable facts.

If you base your decisions on the target performance, do you really care about debates on fuzzy values like touch points? As long as you have that reference round and base your ammo seating depth from that tool, you should be able to make progress.

Good Luck and in for the range reports.

 

[straightshooter1]

Okay maybe I am missing something or not explaining properly.. My goal is to stay 10 of the lands with the new 0.006 larger case.. I assumed (and I can be wrong) that the 0.006 expansion is neck down so the case is longer from base to neck by 0.006 so when seating a bullet I need to seat 0.006 longer to maintain that 10 off the lands distance.

I can be completely off.

And yes 100% agree with pressure, etc if I were to bring in 4 tenths off the lands. (Which is what I am trying to avoid)

Mike

Though your case shoulder moves .006" shouldn't affect your seating of the bullet. When seating a bullet, your seating pin pushes on a high point of the ogive and the whole cartridge is pushing against the base. The position of the shoulder doesn't come into play. Using your same set up on your seating die after the shoulder expands will produce the same BTO, therefore you'll still be at .010" off the lands.

Assuming the case's have the same OAL after trimming, if the shoulder is .006" more, this means the length of the neck is .006" less. Therefore, there's less of a neck on the bullet's bearing surface (less interference, given "neck tension" is the same).

. . . unless you have some different seating die, that I've never heard of, that uses the shoulder instead of the base to press against when seating a bullet???

 

[Doom]

I'll give a slightly different idea. Keep the same CBTO and see how it shoots. Then work any issues from there. There are several other variables at play but it is best to use fired brass as your basis.

 

[CanMike]

Ya I think I’ll start a ok t get velocity data with powder and fine tune the seating depth.

I assume the seating depth or distance to lands is influenced by shoulder not case length. As the brass is pushed into chamber it stops at shoulder. Case can be longer but it will always stop at shoulder.

In my guess the cases is slightly shorter OAL then virgin brass but shoulder is down is 0.006 longer. Case definetly blew out during firing as I think the virgin lapua brass is on the smaller side of SPEC.

Mike

 

[Bullseye Smith]

Though your case shoulder moves .006" shouldn't affect your seating of the bullet. When seating a bullet, your seating pin pushes on a high point of the ogive and the whole cartridge is pushing against the base. The position of the shoulder doesn't come into play. Using your same set up on your seating die after the shoulder expands will produce the same BTO, therefore you'll still be at .010" off the lands.

Assuming the case's have the same OAL after trimming, if the shoulder is .006" more, this means the length of the neck is .006" less. Therefore, there's less of a neck on the bullet's bearing surface (less interference, given "neck tension" is the same).

. . . unless you have some different seating die, that I've never heard of, that uses the shoulder instead of the base to press against when seating a bullet???

That is what I said in a statement a few places up. You broke it down more to understand.

 

[vh20]

Wouldn't it be right to keep the die set the same for new brass and that would be the bullet is only set .006 deeper into the brass. Then the overall measurement would be 2.200 which is .010 off the lands! Then trim after two loads or three.

This is the answer to his question. Whether or not it will give him optimal results with the change in other case dimensions that have happened due to the first firing is still unknown, but your answer is correct to what he asked. (It's also what I would do if I were him, then re-adjust if performance wasn't the same, but if seating depth was best for him at 0.010-off, it probably still will be).

 

[straightshooter1]

That is what I said in a statement a few places up. You broke it down more to understand.

Yup, it was to be in support of what you said. . . AND to hopefully make it a little easier to understand.