Distance to Lands vs Seating Depth Question

[Phil3]

I have read and heard from others two pieces of advice that conflict. Looking for clarity.

Some say distance to the lands is all important, and that with throat erosion, one must continually move the bullet out of the case to maintain the original distance to lands. Others say seating depth in the case is all important, and once determined, it stays there, regardless of the lands moving further and further away due to throat erosion. It is obviously impossible to maintain a fixed distance to lands and a fixed seating depth, since the lands are continually moving away due to erosion.

The question is which one (seating depth or distance to lands) is most critical to accuracy? Maybe both? Maybe "it depends"? Since I've no idea, that is why I am asking here.

Thanks,

Phil

 

[Bill Norris]

Well, u might expect this answer. It will depend on your barrel. Some barrels will shoot consistently good groups the life of the barrel without ever changing seating depth and some will want to be moved but the node will determine your accuracy not distance to lands. Erik Cortina has a video on you tube that explains it very well. He says chase the node not the lands.

 

[Phil3]

Well, u might expect this answer. It will depend on your barrel. Some barrels will shoot consistently good groups the life of the barrel without ever changing seating depth and some will want to be moved but the node will determine your accuracy not distance to lands. Erik Cortina has a video on you tube that explains it very well. He says chase the node not the lands.

Thanks. I have watched Erik's video, and many of his other's, and he does state what you say. I was just curious from a technical point of view which (chasing lands vs seating depth), would likely have more of an impact on accuracy. I recognize there may be no good way to know for sure, or that it is simply determined by a multitude of factors too numerous to list and/or test.

Phil

 

[NorCalMikie]

???? That's the reason I start off with ZERO freebore when I order a reamer.
I only bench shoot and single load so my loads are built with the lightest bullets I can find, stuffed .010 into the lands.
As the throat wears I can increase the OAL and when needed, I'll go to a heavier bullet.
Nothing worse than finding YOUR sweet spot, then it goes away cause the bullet won't reach the lands anymore.
I know where the lands are and where each weight bullet sits to a jam.
Don't like a jam into the lands? Jump and see how that works for you.
What works for me may not work for you. It's all a guessing game anyway.

And I'll add, with the bullets jammed into the lands, I don't see pressure. That exact same load with the OAL set to a jump "will" show pressure. Go back and jam into the lands and "no" pressure signs. Something to do with a pressure spike? All I know is it works for me.

 

[L.Sherm]

Are you Target, BR or hunting, the latter im not jamming

 

[Phil3]

Are you Target, BR or hunting, the latter im not jamming

Good question! Target shooting off bench or prone, sometimes with mechanical rest, other times a bipod and rear bag.

 

[Straightshooter1]

Thanks. I have watched Erik's video, and many of his other's, and he does state what you say. I was just curious from a technical point of view which (chasing lands vs seating depth), would likely have more of an impact on accuracy. I recognize there may be no good way to know for sure, or that it is simply determined by a multitude of factors too numerous to list and/or test.

Phil

In my limited experience, I found that seating depth much much more critical to accuracy than the jump to the lands. My experience is only with my .308 and 6.5 PRC bolt guns with Krieger barrels. Once I found a seating depth that is producing accurate results (usually relatively close the the lands), I stay there. When I chase the lands I'm changing seating depths which means the case volume for the particular powder charge is changing and making those changes in volume can really upset the accuracy node one has found. Eventually, some change in seating depth will need to be made along with the powder in order to get back into the accuracy node. The type of bullet being used can be a factor too as some bullets just don't like much jump and it may be a bigger factor than with the other bullets. As an example, on my .308 with 168 SMK's I've gone well over a 1,200 rounds without loosing accuracy as the lands had moved over .030". And I'm talking my accuracy staying well under .5MOA.

If you haven't read the articles over at Precision Rifle Bog.com regarding bullet jump, you should read them and you can see that jump is not as big a deal as some make it out to be: https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/

 

[Phil3]

In my limited experience, I found that seating depth much much more critical to accuracy than the jump to the lands. My experience is only with my .308 and 6.5 PRC bolt guns with Krieger barrels. Once I found a seating depth that is producing accurate results (usually relatively close the the lands). When I chase the lands I'm changing seating depths which is means the case volume for the particular powder charge is changing and making those changes in volume can really upset the accuracy node one has found. Eventfully, some change in seating depth will need to be made along with the powder in order to get back into the accuracy node. The type of bullet being used can be a factor too as some bullets just don't like much jump and it may be a bigger factor than with the other bullets. As an example, on my .308 with 168 SMK's I've gone well over a 1,200 rounds without loosing accuracy as the lands had moved over .030". And I'm talking my accuracy staying well under .5MOA.

If you haven't ready the articles over at Precision Rifle Bog.com regarding bullet jump, you read them and you can see that jump is not as big a deal as some make it out to be: https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/

Thank you. Great info, and now, I am headed to the link you provided. - Phil

 

[garandman]

Aren't seating depths and jump to lands 2 different ways of discussing the same concept ?

When I find my distance to lands, I determine my seating depth by how far off the lands I want to be.


???

 

[Straightshooter1]

Aren't seating depths and jump to lands 2 different ways of discussing the same concept ?

When I find my distance to lands, I determine my seating depth by how far off the lands I want to be.

Yes.

When finding a good accuracy node with a certain distance off the lands, then that seating depth is the main thing that's giving your gun's harmonic frequency timing for accuracy. If you chase the lands, you change the seating depths resulting in a change in that harmonic frequency timing that the cartridge's load produces.

 

[Bill Norris]

Yes.

When finds a good accuracy node with a certain distance off the lands, then that seating depth is the main thing that's giving your gun's harmonic frequency timing for accuracy. If you chase the lands, you change the seating depths resulting in a change in that harmonic frequency timing that the cartridge's load produces.

Makes perfect sense. And that harmonic frequency timing travels in a bell curve with some being wide at the accuracy point and some being narrow at accuracy point which produces a different width of small groups vs. larger groups at the top of each curve.

 

[Phil3]

Aren't seating depths and jump to lands 2 different ways of discussing the same concept ?

When I find my distance to lands, I determine my seating depth by how far off the lands I want to be.


???

True, but my original point was, what does one do when the lands move (erode)? Leave the bullet at the same seating depth (while lands move away), or chase the lands and change the seating depth?

 

[garandman]

True, but my original point was, what does one do when the lands move (erode)? Leave the bullet at the same seating depth (while lands move away), or chase the lands and change the seating depth?

Aaaahhh... OK.

I've never shot out the lands of a barrel to have them erode that much. So no personal experience.

If my groups didn't Open up, I'd leave it alone.

If my groups opened up I'd find the new distance to lands and seat to the same jump.

If that didn't work I'd Go through the usual seating depths tests to find the optimized jump.

 

[Bill Norris]

If accuracy falls off or after @ 300 rounds fired test .003 longer than your established accuracy depth to see if it improves. This is what Erik means when he says chase the node rather than the lands.

 

[garandman]

If accuracy falls off or after @ 300 rounds fired test .003 longer than your established accuracy depth to see if it improves. This is what Erik means when he says chase the node rather than the lands.

Yup. Measuring the size of the group on the target matters more than measuring the jump to the lands.

 

[Bill Norris]

Yup. Measuring the size of the group on the target matters more than measuring the jump to the lands.

Exactly.

 

[BoydAllen]

If you establish a powder charge and seating depth that give best accuracy, it should be understood that that may change as the barrel wears, which logically should dictate a minor retune. Experimentation will show what the most effective way to do this. Erik did not say not to change your seating depth. What he did say was that you need to pay attention to accuracy and let that dictate when to change and how much. Seating depth is usually expressed relative to the lands. Shooters will say that they seat their bullets to jump (to the lands) a particular distance, or to touch, or some distance into the lands, meaning that much longer than the length that just touches the rifling. In the short range game, I establish a load which has typically included bullets being seated some distance into the rifling. As the throat wears, and that relationship to the lands changes, typically I have seen accuracy suffer. Seating the bullet farther out of the case to compensate has restored the lost accuracy. I have seen where a move of .002 was all that was required to restore accuracy. If you are trying to learn this by any other means than doing your own experimenting, IMO you are never going to get a feel for this.

 

[CNC-Dude]

Erik also explains that the relationship to the lands is not a one to one ratio, the seating depth and powder charge for that depth creates the velocity node, not the distance to the lands. So if you start chasing the lands, you will take yourself out of the node that is working into one that more than likely doesn't. And you have to begin load development all over again.

 

[243winxb]

Leave the bullet at the same seating depth

This.
Been shooting same Rem 40x in 243 since 1976. I started a log book late, but more then* 2000 rounds fired using same Sierra # 1530 with IMR 4350. Same COL. Yes, groups have gotten 1/4" bigger with this bullet. Can no longer jam bullet into rifling, like when new. Lots of jump now.

After getting online in 2000, reading accuracy report, i put a 36x scope on the old gun.

Berger bullets - a hair under 1" @ 300 yards, at best, 5 shot groups. 68gr FB & 90 gr hpbt. When wind blows & mirage starts, groups get a lot bigger, closer to 2.75"

Grandson can hit an egg @ 300 yards with it.

 

[ELR LVR]

True, but my original point was, what does one do when the lands move (erode)? Leave the bullet at the same seating depth (while lands move away), or chase the lands and change the seating depth?

Depends on how much your throat has eroded and at what rate your throat erodes.
Barrel steel plays a role here.
IE: I think a Douglas is rather soft hence eroding more quickly (My 22-250 has .300" throat erosion)
---While a Krieger is much more wear resistant. (My 6x284 has no measureable throat erosion after 700 rnds)
I have read that (Cartridge dependent of course) erosion rate is approx. .0001" per 10 shots
or .001" per 100
If your throat has only eroded .010" in 1000 shots then it "May" be time to just bump the seating depth out a little.
The barrel harmonics may not like this new seating depth though.
---Part of the reason for chasing the lands is to prevent as much gas from escaping around and in front of the bullet since not only the lands but also the freebore section wears and enlarges slightly which reduces pressure and can cause higher ES in velocity and undesireable SD's.
So after a certain amount of wear I would seat the bullet out further mostly to keep a certain desired velocity up yes.
You may even have to go with a longer bullet to do so if your twist rate will allow it.