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who does this method? Chasing Lands

I just watched Eric Cortina explain why chasing your lands means nothing. Must admit ,

In short, he suggested to Jam your lands back off .020. Then load 5 shots groups ( could be 3) and keep backing off by .003

You will get large group, large group, a better group then maybe 2 or 3 groups that are tight. That is your best seating depth and it means nothing with respect to the lands? He claims you need multiple tight groups that are .003 apart before you settle. Then take the longest AOL and subtract .001. Has anyone done this type of testing?


Sorry, I am new to this method and looking for input
What I have found is The harmonics of the barrel don't change in relation to where the lands have eroded to
So yes, in my opinion Eric is correct, Why chase the lands if you've found your magic seating depth?
(This may be totally different however if there is a lot of erosion and say now you're suddenly jumping .200" in which case the pressures and velocity behavior may change with the same load compared to before when the throat was newer, in this particular circumstance then, changing seating depth may help to correct load characteristics)
But once I have found the right seating depth, it stays the same for well over 1000 rounds in any rifle.
-------------------
Some people may get it into their mind that since their rifle shot best .020" off the lands....
......They need to stay .020" off the lands as the throat wears.
Mt. Cortina is helping to correct this type of false thinking
 
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I just watched Eric Cortina explain why chasing your lands means nothing. Must admit ,

In short, he suggested to Jam your lands back off .020. Then load 5 shots groups ( could be 3) and keep backing off by .003

You will get large group, large group, a better group then maybe 2 or 3 groups that are tight. That is your best seating depth and it means nothing with respect to the lands? He claims you need multiple tight groups that are .003 apart before you settle. Then take the longest AOL and subtract .001. Has anyone done this type of testing?


Sorry, I am new to this method and looking for input
Yes, I do this method all time time and it has work well for load development for my .308 and 6.5 PRC that I precision load for.

Note: though I will load up 5 cartridges for each powder charge (.3 grs apart) and for the following seating depth test at .003" apart. Typically, I start at .010 off the lands for this testing. But, when loading for cartridges to fit in a mag, how far you are from the lands isn't a factor. . . unless you have a really short freebore. I don't always shoot all 5 cartridges for a load, where if I see the first 2 shots far apart, I know I further shots are not gong to improve the size of the group, so I move on.

In a lot of instances, the problem is being able to interpret the target results. It's best to shoot the groups along a horizontal line to see how their position compares to the others in addition to the shape and size of the groups. Not having good shooting skills can get in the way of understanding what is seen on the target.


What I have found is The harmonics of the barrel don't change in relation to where the lands have eroded to
So yes, in my opinion Eric is correct, Why chase the lands if you've found your magic seating depth?
(This may be totally different however if there is a lot of erosion and say now you're suddenly jumping .200" in which case the pressures and velocity behavior may change with the same load compared to before when the throat was newer, in this particular circumstance then, changing seating depth may help to correct load characteristics)
But once I have found the right seating depth, it stays the same for well over 1000 rounds in any rifle.
-------------------
Some people may get it into their mind that since their rifle shot best .020" off the lands....
......They need to stay .020" off the lands as the throat wears.
Mt. Cortina is helping to correct this type of false thinking
Likewise, I've gone over 1,000 rounds (actually ~2,000 rounds in my .308) with no changes to the seating depth where the accuracy and precision maintained quite well. . . even though the throat eroded ~.032" over that time.

I think you right . . .the harmonics stays the same, which has to do with barrel exit timing. But what also stays pretty much the same pressure behind the bullet as it begins to move and the timing for the blowby as the gases get past the bearing surface.

Because the chamber volume increases as the throat erodes due to where the bullet becomes fully engraved, eventually that's going to change the the barrel exit time where one will need to make a small adjustment either in seating depth or powder charge. After 2,000 rounds out of my .308 I didn't make a small change in powder charge to tighten things up a bit. No need to chase the lands. ;)

I wish guys would state if they are competition shooters or casual shooter/hunters with OCD
I'm not a competition shooter. . . just someone with a strong perfectionist tendency. :p
 
It's best to shoot the groups along a horizontal line to see how their position compares to the others in addition to the shape and size of the groups. Not having good shooting skills can get in the way of understanding what is seen on the target.



Likewise, I've gone over 1,000 rounds (actually ~2,000 rounds in my .308) with no changes to the seating depth where the accuracy and precision maintained quite well. . . even though the throat eroded ~.032" over that time.

I think you right . . .the harmonics stays the same, which has to do with barrel exit timing. But what also stays pretty much the same pressure behind the bullet as it begins to move and the timing for the blowby as the gases get past the bearing surface.

Because the chamber volume increases as the throat erodes due to where the bullet becomes fully engraved, eventually that's going to change the the barrel exit time where one will need to make a small adjustment either in seating depth or powder charge. After 2,000 rounds out of my .308 I didn't make a small change in powder charge to tighten things up a bit. No need to chase the lands. ;)


I'm not a competition shooter. . . just someone with a strong perfectionist tendency. :p
Very much agreeing with your results in testing with what you said here. And like what you said about shooting groups along a horizontal line to be able to more closely compare how different loads react along the same waterline.

And yes, as throat erodes, once at a certain point I agree making an adjustment in charge weight to make up for velocity loss, may likely be all that is needed to compensate....as opposed to making a seating depth change.
I say "Likely" since as I said earlier what if we haven't adjusted anything yet until say .200" throat erosion has happened?
it may need a bump in charge as well as a seating depth change to retime the bullet exit.
These are more extreme circumstances in my opinion.
------------------------------------
For the most part though, it looks like most people are seeing that a seating depth change is not necessary until....the barrel has significant wear anyway, in which case I think we're just operating on its last legs needing a new barrel pretty soon by that point, or a chamber refresh and just trying to get the most out of it until then.
 
I just feel that you can call it what you will, but there is absolutely a relationship between the seating depth and the lands. Whether you call it so far off the lands, or CBTO, you will still be talking about some type of relationship between seating depth and the lands.

When I set up my seating die, I use a reference cartridge that I made which is touching the lands. I know that I need to set up my die to be so many thousands off the reference cartridge. So at that point is it CBTO, or distance to the lands. Call it what you will, it comes out the same.
 
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This is another reason I set barrels back on a consistent bases. Usually about every 350 rounds.

I feel that keeping the throat as fresh as possible is important in keeping a barrel, bullet, load combo shooting at it’s best.

I know this is not practical for those that do not do their own barrel work. But for those that do, it is not that difficult if you do your barrels in a manner that lets you set it back up in the exact same position in your lathe as when you first chambered the barrel.
 
I take a measurement on a new chamber , and adjust it in from there . Then ; It stays there until I see a drop in accuracy and X-count , or normal average score . Usually around 1,200 - 1,500 rounds . I re-set to the "new" measurement , and do a slight seating depth adjustment , and go again . It's a .308 , so it doesn't have to be moved as often . Maybe twice in the life of the barrel ? If it ain't broke , stop trying to fix it !
 

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