Chasing lands

[Aggrofish]

So my 6.5CM has about 1800 rounds through it and the velocity has been decreasing lately. Same brass/primer/powder.

With the Hornady 140 ELDM the CBTO was 2.258 and I was seating .020 off, or 2.238. It’s been a very accurate load. Today I measured and the CBTO was 2.293 or .035 difference (erosion). I’m going to load some rounds and reseat to .020 off and probably add a couple 10ths to get the velocity back.

Question: is .035 a lot or a little?
Also, does seating depth or jump affect velocity? From what I read so say yes some say no.

 

[918v]

Tell us more about your case prep

 

[straightshooter1]

So my 6.5CM has about 1800 rounds through it and the velocity has been decreasing lately. Same brass/primer/powder.

With the Hornady 140 ELDM the CBTO was 2.258 and I was seating .020 off, or 2.238. It’s been a very accurate load. Today I measured and the CBTO was 2.293 or .035 difference (erosion). I’m going to load some rounds and reseat to .020 off and probably add a couple 10ths to get the velocity back.

Question: is .035 a lot or a little?
Also, does seating depth or jump affect velocity? From what I read so say yes some say no. View attachment 8440449View attachment 8440460

The important thing is, are you really seeing a difference on paper?

There can be a number of things effecting the difference in velocity, like simply wear on the bore. What make you think is the change in jump?

For my .308, I kept the seating depth constant at the throat eroded ~.034" from just a hair over 2,000 rounds. I was beginning to see a difference on target, so I adjusted my tuner and brought it back where I felt it should be without changing the seating depth. In your case, with .035" of throat erosion, you may indeed need to make a small adjustment to your seating depth. But, going back .020 off the lands is mostly likely not the thing to do.

And yes, seating depth does effect velocity. But jump has very little effect if any. You should find, when doing load development, that as you seat deeper into the case, velocity will decrease. So, seating further out of the case it will tend to increase velocity. But if you try to increase your velocity this way, you could very well loose the tune of your load and not get the results on paper you've been getting.

With 1800 rounds through the barrel, I'm leaning that the increase in velocity is mostly due to the wear on the bore.

 

[Aggrofish]

Tell us more about your case prep

Same as it's alway been. Anneal, size w/ redding FL busing die, .002 neck tension, tumble, chamfer / deburr. Rarely trim as the Lapua cases don't stretch much.

 

[918v]

Stop annealing. If you overdo that then the case gets too soft and velocity drops.

 

[Aggrofish]

The important thing is, are you really seeing a difference on paper?

There can be a number of things effecting the difference in velocity, like simply wear on the bore. What make you think is the change in jump?

For my .308, I kept the seating depth constant at the throat eroded ~.034" from just a hair over 2,000 rounds. I was beginning to see a difference on target, so I adjusted my tuner and brought it back where I felt it should be without changing the seating depth. In your case, with .035" of throat erosion, you may indeed need to make a small adjustment to your seating depth. But, going back .020 off the lands is mostly likely not the thing to do.

And yes, seating depth does effect velocity. But jump has very little effect if any. You should find, when doing load development, that as you seat deeper into the case, velocity will decrease. So, seating further out of the case it will tend to increase velocity. But if you try to increase your velocity this way, you could very well loose the tune of your load and not get the results on paper you've been getting.

With 1800 rounds through the barrel, I'm leaning that the increase in velocity is mostly due to the wear on the bore.

The difference in velocity is causing all my calculations to be off past 1000. I forget if it was 1100 or 1250 today but I had to dial up another .7 mil which I've never had to do. I know the barrel is getting worn which is fine.

 

[Aggrofish]

Stop annealing. If you overdo that then the case gets too soft and velocity drops.

interesting, but if you look at the garmin all those cases were from the same batch of 200 pieces all 3x fired and annealed.

 

[straightshooter1]

Stop annealing. If you overdo that then the case gets too soft and velocity drops.

What do you mean by "overdo"?

Are you suggesting that just repeated annealing (when done correctly) is what'll make brass too soft? If so, that's simply not true.

 

[918v]

interesting, but if you look at the garmin all those cases were from the same batch of 200 pieces all 3x fired and annealed.

If you don’t anneal for the next 2-3 reload cycles the velocity will creep back up.

 

[918v]

What do you mean by "overdo"?

Are you suggesting that just repeated annealing (when done correctly) is what'll make brass too soft? If so, that's simply not true.

If your chamber is tight enough where firing and subsequent sizing isn’t enough to harden the brass to the point where it was, then repeated annealing will soften the case to the point where it is too soft.

 

[straightshooter1]

If your chamber is tight enough where firing and subsequent sizing isn’t enough to harden the brass to the point where it was, then repeated annealing will soften the case to the point where it is too soft.

Sorry, no.

Perhaps this will help convince you:

 

[straightshooter1]

If you don’t anneal for the next 2-3 reload cycles the velocity will creep back up.

Certainly, as interference increases, pressure spike will help velocity increase. Technically, you're right. But, I have doubts OP would regain the velocity he's looking for.

 

[918v]

Certainly, as interference increases, pressure spike will help velocity increase. Technically, you're right. But, I have doubts OP would regain the velocity he's looking for.

1800 rounds on and .035” of erosion on a 6.5cm won’t cause any velocity loss. Allowing the case to harden back to where it was at 1x will restore the velocity.

 

[918v]

Sorry, no.

Perhaps this will help convince you:

I saw that video before. But what he’s saying doesn’t jive with what I’m seeing and feeling when I’m sizing and expanding the case neck and what the chrono says.

What I’m seeing is velocity creeping up as the cases get harder and then falling back to normal after annealing once. If I anneal every cycle the velocity gradually creeps downward and then goes back up when I reload them without annealing a few times.

Certainly it won’t hurt to try. Or load in a 1x factory case to see.

 

[straightshooter1]

I saw that video before. But what he’s saying doesn’t jive with what I’m seeing and feeling when I’m sizing and expanding the case neck and what the chrono says.

What I’m seeing is velocity creeping up as the cases get harder and then falling back to normal after annealing once. If I anneal every cycle the velocity gradually creeps downward and then goes back up when I reload them without annealing a few times.

I'm not disputing what you've observed. But since I've not experienced that phenomenon as I anneal after every firing and metallurgists agree that repeated annealing of brass to the same temp and time does not make the brass softer, I don't have an explanation, though it seems to me something else must going on other than the necks getting softer with each annealing.

Certainly it won’t hurt to try. Or load in a 1x factory case to see.

I agree. . .

It doesn't hurt to look at and try different things to see what's going on.

 

[Cascade Hemi]

I annealed brand new brass last night. I died afterwards.

 

[Aggrofish]

I annealed brand new brass last night. I died afterwards.

RIP

 

[Rocketmandb]

What I’m seeing is velocity creeping up as the cases get harder and then falling back to normal after annealing once.

This makes sense. Think about what's happening. The case is hardening, so it springs back more. More spring-back = more effective neck tension. More effective neck tension = higher pressure. Higher pressure = more velocity.

If I anneal every cycle the velocity gradually creeps downward and then goes back up when I reload them without annealing a few times.

Annealing every cycle won't cause the velocity to continue to go down. You can't over-anneal by annealing properly multiple times - as long as you let the case cool, that is. And yes, I've accidentally annealed the same case twice in a row and over-annealed, but it was because it was hot and I immediately did it again, causing the temperature to go above the target.

Without knowing everything that's going on in your process, it's hard to conjecture as to what's causing what you're seeing.

In the end, it all comes down to the pressure profile, which is driven by burn rate and the forces trying to keep the bullet in place (bullet weight, effective neck tension, seating depth, chamber conditions that might impact such).

I forget, how are you annealing?

 

[Rocketmandb]

I annealed brand new brass last night. I died afterwards.

FWIW, since I'm a glutton for punishment, I tested re-annealing new brass and compared to not doing so. Re-annealed exhibited superior results.

 

[918v]

This makes sense. Think about what's happening. The case is hardening, so it springs back more. More spring-back = more effective neck tension. More effective neck tension = higher pressure. Higher pressure = more velocity.



Annealing every cycle won't cause the velocity to continue to go down. You can't over-anneal by annealing properly multiple times - as long as you let the case cool, that is. And yes, I've accidentally annealed the same case twice in a row and over-annealed, but it was because it was hot and I immediately did it again, causing the temperature to go above the target.

Without knowing everything that's going on in your process, it's hard to conjecture as to what's causing what you're seeing.

In the end, it all comes down to the pressure profile, which is driven by burn rate and the forces trying to keep the bullet in place (bullet weight, effective neck tension, seating depth, chamber conditions that might impact such).

I forget, how are you annealing?

AMP using the Aztec code for the lot.

I use a Lee collet neck die and can feel the neck is getting progressively softer with each cycle. So I don’t buy what he’s saying.

 

[Cascade Hemi]

Remember that time...

FWIW, since I'm a glutton for punishment, I tested re-annealing new brass and compared to not doing so. Re-annealed exhibited superior results.

I'm not sure I would go so far as to say I "tested" it but anecdotally my first-firing SD's are lower if I anneal the brass right out of the package.

 

[375fan]

Remember that time...


I'm not sure I would go so far as to say I "tested" it but anecdotally my first-firing SD's are lower if I anneal the brass right out of the package.

I anneal virgin brass, even Lapua. I'm still alive.

 

[Cascade Hemi]

I anneal virgin brass, even Lapua. I'm still alive.

You sound a little dead.

 

[Rocketmandb]

I anneal virgin brass, even Lapua. I'm still alive.

You have to make sure your quantum flux coefficient stays within acceptable boundaries or the universe will implode.