Help with the concept of 'chasing the lands'

[Mike402]

I am new to the 6mm world and am running a 28' Bartlein slinging 115 dtacs. I am currently at ~ 600 rounds, and have been surprised at how much the throat is moving. I am measuring every 100 rounds, and seeing consistent erosion. Also seeing firecracking in the throat at this point.

Here is my question - what is your procedure for chasing the lands? I understand seating the bullets a little longer to account for the wear, but do you adjust your powder charge each time? Of course it wouldn't make sense to redo load development each time, and wear the barrel out even quicker. How do you determine the ratio of adding powder for each corresponding change in COAL as you chase the lands? Doesn't the pressure change each time and throw you off your node?

Thanks in advance...

 

[Fast14riot]

It is my understanding that distance to the lands is the primary cause of pressure change, therefore adjusting seating depth to follow the lands every so often is required. I have not tried adjusting charge weight to compensate for the increased case capacity, although finding a nice stable node allows for variation without significant change in POI.

 

[dmoran]

@Mike402
For the most part, I start out by just chasing the advancement with the seating, leaving the charge alone.
After around 0.020" or so of advancement, I do take a look at the charge again, re-finely. Maybe change a tenth but likely no more then 2-tenths, if at all.
Then around 0.040" of advancement, I seriously consider and am likely to re-chamber. When I re-chamber, I have them set them back at least 1", and also re-crown going in at least 0.025" or so.
My 2-Cents
Donovan

 

[Richard Coody]

Well i shoot most of my target rifles jammed .010 to .015 so it takes a while for throat erosion to cause a deterioration in accuracy. That is your clue. Deterioration in accuracy. If you are jumping and have a narrow accuracy node on your seating depth that deterioration will show up sooner.

Another easy way to avoid it is with a soft jam. Then you let the rifle bolt do the final.seating.

My hunting rifles i jump. But i don't shoot them enough to worry about erosion.

 

[watercam]

It is more critical with VLD bullets than any alternative. I check every 500 rounds and adjust accordingly. One would have to have quite a change to affect pressure - .025" or more. Even at that I would bet you would not see a significant velocity change that could not be explained by atmospheric conditions as well. For my .308 Palma barrel I am seeing less than .010" change in 500 rounds. My 6XC might change by .015" at the most in the same round count. My .223 Palma barrel has not changed in 1000 rounds so far.
David Tubb recommended polishing the throat when fire cracking appears periodically. I have done so at the same 500 round interval.

 

[Mike402]

Thank you for the replies. I am trying to avoid jamming, as I really want to avoid getting a bullet stuck in chamber and pouring powder all over my action. I mostly go to long range classes, and am looking into prs type events.

So it sounds like best way to proceed is to simply seat a little longer and keep an eye on velocity. If velocity goes under a certain threshold, then that would be the time to add powder to compensate

 

[Mike402]

David Tubb recommended polishing the throat when fire cracking appears periodically. I have done so at the same 500 round interval.

And the best way to do that would be by using his throat polishing bullets? I'm apprehensive about causing even more erosion

 

[uncle-buck]

@Mike402
For the most party, I start out just by chasing the advancement with the seating, leaving the charge alone.
After around 0.020" or so of advancement, I do take a look at the charge again, re-finely. Maybe change a tenth but likely no more then 2-tenths, if at all.
Then around 0.040" of advancement, I seriously consider and am likely to re-chamber. When I re-chamber, I have them set them back at least 1", and also re-crown going in at least 0.025" or so.
My 2-Cents
Donovan

What do you mean by "set them back at least 1" ?

 

[dmoran]

@Mike402 -
Myself would only use "fire lapping bullets" in rough factory barrels, and would never consider using them in a aftermarket hand-lapped match barrel.
Donovan

 

[dmoran]

What do you mean by "set them back at least 1" ?

Cutting off the chamber/breach end at least 1" and re-fit/chamber. Hence, setting it back at least 1".
Donovan

 

[memilanuk]

@Mike402
For the most party, I start out just by chasing the advancement with the seating, leaving the charge alone.
After around 0.020" or so of advancement, I do take a look at the charge again, re-finely. Maybe change a tenth but likely no more then 2-tenths, if at all.

Donovan,

How much does that formula vary by caliber/cartridge? i.e. 6 Dasher vs. .308 Win vs. .300WM...

 

[dmoran]

@milanuk -
Good point... my references have all been from 243/6mm calibers/barrels and for benchrest type accuracy.
Can't speak for other calibers and scenario's. Sorry, should have included that above.
Donovan

 

[Delfuego]

I am new to the 6mm world and am running a 28' Bartlein slinging 115 dtacs. I am currently at ~ 600 rounds, and have been surprised at how much the throat is moving. I am measuring every 100 rounds, and seeing consistent erosion. Also seeing firecracking in the throat at this point.

Big questions would be, which caliber? What speed? Which Powder?

 

[ShootDots]

I will give you a good example. My "new" .260A.I. barrel ( I have 2 /.260A.I.'s) is using H4350. I finally found some RL-16 and RL-26 and am presently in the process of developing a load with it (RL-26). WHY? Excellent question! I just finished a match 3x20 + sighters and fouling rounds (100 rounds total)... The throat moved 8 THOUSANDSTHS! That is horrible! So no, I will not change the powder charge. However, I must move my bullets out 8K to stay at the correct seating depth. Will 8K erosion in and of itself necessitate an increase in powder>>no... On the other hand, If I end up having to use H4350 for too many more matches, you can bet it will! That is why I have decided to switch to another powder... However, at the inception of that barrel, there was no RL-16 nor RL-26 to be had. I have a decent supply of H4350, so out of desperation I went with it. Now H4350 is one of the finest powders out there for the application I was using it in>>>>but it eats throats up like candy! You merely have to "chase the lands"!

 

[NorCalMikie]

It has something to do with pressure spike. Don't know all the in's and out's but I load the way it works best for me. YMMV.

I set all my loads (except my gas guns) with the bullets at .010 into the lands. Reason being?
One load, .010 from the lands will start showing pressure.
Same load with no other changes except the OAL, bullets set at .010 into the lands, won't show pressure and you can even go higher on the powder charge without any problems.
Mind you, this is for bench shooting. I have opened the bolt on a loaded round with the bullet stuck in the lands and dumped powder into the action. More than once. I know it's coming so I'am ready for it, kinda.
On an initial start up, check the distance to the lands and load accordingly. Check it often.
I check each bullet style I use, mark it down and refer to my chart often. You would be surprised just how fast the lands wear.
That's the reason ALL my my chambers are custom cut with ZERO freebore. I start off with lighter bullets stuffed into the lands and go heavier as the throat wears. You start off with .100 freebore, you're fine for a while and in short order, you can't reach the lands with your favorite bullets. Made that mistake starting out with an "off the shelf" chamber. Won't do that again.
Larger calibers can go with more freebore. Smaller calibers can be stuffed!

 

[MagnumManiac]

I use to chase the lands in my comp rifle, now I wait for accuracy degradation then adjust the POWDER CHARGE to get the same velocity as before.
I can go a whole match season without chasing the lands if I adjust charge only and setback one turn and re-chamber. Previously, I was chasing, re-chambering every 400 rounds down with single base powders.
I also noted that using double base powders reduced throat erosion to almost half in the 300WM barrels I was using. The haze cracking was still there, to be expected, but the overall erosion was less.

Barrel time plays a far bigger role in precision than does the seating depth, if you keep the velocity the same, within reason, the outcome is nearly always in your favour. I found numerous times that seating depth change alone did not get me back in the node as easily as charge weight/velocity did.

Cheers.

 

[pacificman]

When finding a load using the OCW method you will have some leeway as you generally pick a load in the middle of three
groups of five shots that have the same point of impact....so with throat erosion I can seat my bullets closer to the lands knowing I should still be in the accuracy node. Probably a good idea to check velocity and target though.

 

[Raythemanroe]

Every hundred to two hundred rounds Take out the fire control and ejector and find your touch again. This will tell you how much you need to chase..


Ray

 

[Mike402]

Big questions would be, which caliber? What speed? Which Powder?

Caliber is .243 (I knew going in it was a barrel burner), Started out with H4350, but then tried RL26. Got an additional 100 fps & good accuracy/ES/SD, so that was the last 300 rounds. Shooting the newer DTAC RBT 115s at 3050.

I can't find any more RL26, so going back to H4350 since I have an ample supply of it. I know I might possibly retard erosion by going with a slower powder, but I have way too much invested in powder, and need to use up what I have on hand.

I bought two of these barrels at the same time anticipating this dilemma.

However, since I am not currently competing and actually witnessing how fast this erosion takes place, I'll probably move to a 6.5 variant and make my life a little easier next time around. Also tired of trying to catch the DTACs in stock at David's site.

 

[Mike402]

By the way, my throat has eroded .018 in 600 rounds