6BR throat errosion question.

[Denis Aarons]

G'daapy New Year guys,

Tonight I thought I'd re-check the land position on my 6BR.  Its been approx 300 rnds since I last checked.  
My load is on the heavier side I believe, at 30.4g AR2208, 105g Berger VLD's and CCI 450 magnum primers.
It looks as though my lands have moved back (assuming throat errosion) by approx .007".  Does that sound like the correct rate for SS target barrel in this situation or is it more likely that I've made an error somewhere?

Im pretty confident that my measurements have been correct on all occassions.

Again Happy New Year, I hope to meet some of you this year and hopefully swat a few flies.

Thanks guys, Denis.

[Denis Aarons]

Sorry i should add that shooting F-class I generally take 10 - 15 minutes to shoot my 12 rounds, so Im not "machine gunning" them.  Maybe a couple if I've got a consistant wind condition.

[Pete van Meurs]

Denis,

This is probably somewhere near the mark. Don't forget that the relatively sharp little edges of a newly cut chamber's throat will be the first to be eroded (actually molten off). Some guys keep altering their seating depth to maintain the same "jam" or "jump" depending on preference. I have discovered that starting with .010" jam and no change to the seater that after 1500 rounds the throat eroding to the point where it now has .025" jump with no change in accuracy!!!  There is a reduction in velocity  but while the accuracy remains I do not change the load either. My original 6mm Dasher (Red 7) was a match winner from day one right up to it's retirement after about 2000 rounds without changing anything (last match was it's lg win at 2011 Federal Cup).

Pete

[Denis Aarons]

Hi Pete,

What a great answer, exactly what I wanted to hear with a great example too.  This is my first experience with throat errosion.  I think I might start checking the lands every 200 rnds or so just to keep myself in the loop.

I'll do a comparison with a batch at my current COAL and a batch jammed .005" again which is where they were showing the tightest groups.  
I think from the last couple of shoots that the groups have opened a little (slightly), I was thinking that it was me.  Thinking about it now though, the pattern is very similar to the closer loads when I first developed for seating depth.

Cheers Pete, its always great to get some solid advice based on the personal experience of a pro.

Denis.

[Anthony Hall]

Hi Dennis,

I would think Pete is exactly on the money with his response.... what you may be experiencing is something that I found.

I believe that most loads work either best jammed or jumped, but when you get too fine with either they open up.  If you started with 0.005" jammed and the lands moved 0.007 there is a period where some are jumped and some are touching etc...

In my opinion, you need 0.010" + either jumped or jammed to allow slight variations in seating depth and or even carbon on the lands between cleans etc not to affect the pressure and therefore velocity etc.

Obviously with Petes example of not changing a thing for 2000 rounds, there is only a short period where tolerances are close while it transitions from jammed to jumped.

Cheers

Anthony

[Denis Aarons]

Hi Anthony,

Thanks for adding.  That makes perfect sence.

This may be B.S and please correct me if I'm wrong, but I suspect that a quality custom barrel may be more harmonically stable than a factory barrel.  The reason I'm thinking along those lines is because you guys are shootings custom barrels and can maintain 1 COAL through such a large amount of land movement and still group tightly.
My 30" SS factory Savage target profile barrel really only liked to shoot one little hole at .005" jammed.  

.005" +/- .002" would make 2 holes very close together.  3 in one and 2 in the other, which I suspect is 2 different harmonic nodes as the bullet exits the muzzle.  Still under .5MOA at 100m.

Anything different to those COAL caused consistant horizontal and vertical fliers and groups opened up every time.....gauranteed.

I'm aware it comes with its dangers, but I've read of high power shooters in the States "soft seating".  Does any one here do that.  That would eliminate the need to monitor the lands regularly and stay on them, allowing for carbon build up too.    

Cheers,

Denis.