Barrel throat erosion and accuracy.

[jlow]

So it is pretty much a generally accepted conventional wisdom that the lost in barrel accuracy due to wear is due mostly to throat erosion. The rationale being that as the throat erodes; you are basically increasing the jump distance for the bullet so moving out of that sweet seating depth spot. The question is if that is true, would not jump tolerant bullets like the 175 SMK be less affected by throat erosion?

Now barrel erosion is certainly not limited to the throat area, but would it be true that area is the “first to go?”

 

[ShootDots]

So it is pretty much a generally accepted conventional wisdom that the lost in barrel accuracy due to wear is due mostly to throat erosion. The rationale being that as the throat erodes; you are basically increasing the jump distance for the bullet so moving out of that sweet seating depth spot. The question is if that is true, would not jump tolerant bullets like the 175 SMK be less affected by throat erosion?

Now barrel erosion is certainly not limited to the throat area, but would it be true that area is the “first to go?”

Certainly throat erosion plays a significant role in the deterioration of accuracy. However, I believe that firecracking and the concomitant build up of both carbon and copper in that portion of the barrel loaded with firecracking, is the larger culprit in the decay of accuracy. However, you almost can't have one without the other.

 

[jlow]

Right – as I mentioned in the OP, throat erosion is certainly not the only thing that is happening. The question still is what is the “rate limiting step” or is there one? The reason I ask is because if throat erosion is the major reason initially for lost of accuracy, you can see that a jump tolerant bullet would be resistant to this or at the very least, resistant to this part of the “wear factor”.

It is also true that unlike throat erosion and firecracking, carbon and copper build up does not have to happen - that is a function of how one cleans.

 

[ShootDots]

Right – as I mentioned in the OP, throat erosion is certainly not the only thing that is happening. The question still is what is the “rate limiting step” or is there one? The reason I ask is because if throat erosion is the major reason initially for lost of accuracy, you can see that a jump tolerant bullet would be resistant to this or at the very least, resistant to this part of the “wear factor”.

It is also true that unlike throat erosion and firecracking, carbon and copper build up does not have to happen - that is a function of how one cleans.

You are correct. Cleaning out that carbon and copper lodged in the crevasses created by the firecracking can be done. However, as the firecracking gets worse, the cleaning time increases exponentially! In another thread on here I lightly expounded on a well worn 22-250 I owned. It had in excess of 4000 rounds on it when I finally retired it. Cleaning that barrel was a considerable chore. Even when it was clean, it did not take too many shots to see accuracy fall off. Although it was still pretty accurate. When that rifle was NEW, I used to shoot 52gr Berger M.E.F.'s. Then as the throat eroded, I ended up shooting 55gr Sierra Blitzkings seated just off the lands BUT the bullet was barely in the case. I could reach the proper seating depth for my rifles particular affinity for liking bullets very close to the lands. However, although I could still reach that loading point, accuracy was still W-A-Y off from what it was when it was relatively new. The only other "fly in the accuracy ointment" at the end of that rifles life, was excessive firecracking.

 

[jlow]

Thanks! I know that all barrels wear in terms of throat erosion, but does the degree of firecracking vary between different calibers or does it always go hand-in-hand with the throat erosion?

 

[ShootDots]

Thanks! I know that all barrels wear in terms of throat erosion, but does the degree of firecracking vary between different calibers or does it always go hand-in-hand with the throat erosion?

I think it goes along a different line. ONE: The more OVERBORE you are with an increase in powder volume; TWO: The more towards MAX loads, or in some cases OVER max loads, increasing pressure and heat. THREE: the use of double based powders and FINALLY: Competition! I shoot F-Open in Texas. The heat down here, especially in the heat of the summer, NEVER allows your barrel to cool down. Then here comes match 2 and match 3! The barrel is smokin' hot. I think these are the MAJOR reasons for throat erosion / firecracking. Take a .300R.U.M. and shoot a 3 day F-Open match and I think you will have made toast out of that tube!

 

[jlow]

Very good info - thanks!

 

[ShootDots]

Very good info - thanks!

I hope I answered your question(s) satisfactorily. Hopefully, someone else will chime in with some other point(s) of view..

 

[jlow]

Thanks! You have. I just brought a borescope and was looking at one of my older AR-10 308 barrel and notice a significant degree of throat erosion but nothing that looks like firecracking and so was wondering what the remaining life of the barrel would be? I figured if I shoot it out of a mag, I would not need any great accuracy ~1 MOA would be fine (its doing it now). If I shoot single load, I could probably still set the bullets out long close to the lands and see if I can get better accuracy. Nothing serious, mostly just playing with an old rifle. ;D

 

[ShootDots]

Thanks! You have. I just brought a borescope and was looking at one of my older AR-10 308 barrel and notice a significant degree of throat erosion but nothing that looks like firecracking and so was wondering what the remaining life of the barrel would be? I figured if I shoot it out of a mag, I would not need any great accuracy ~1 MOA would be fine (its doing it now). If I shoot single load, I could probably still set the bullets out long close to the lands and see if I can get better accuracy. Nothing serious, mostly just playing with an old rifle. ;D

The .308 is an interesting animal. It is NOT an overbore, mostly shoots with moderate pressures and normally can get 2500-3000 accurate rounds. However, with the advent of Lapua Palma brass, it is being turned into a pressure cooker. Depending on how accurate Quickload is, along with the information downloaded into it, I know a some F-T/R shooters who are running about 68K P.S.I. pressure. They are shooting 215gr Bergers using copious amounts of Varget in that EXTREMELY strong Palma brass. They are getting unheard of velocities and they are WINNING everything in their path! They are also experiencing rapid barrel wear. Their barrels look (from a Hawkeye) like they are 6.5 x 284's! There is NO FREE LUNCH when it comes to heat and pressure, no matter the cartridge!

 

[jlow]

Yea, this baby has passed the 3k round limit, only an 18” and so not exactly an F-T/R gun but it is a fun gun since it is a semi-auto.

 

[snakepit]

I have been shooting the Berger 105 Hybrids in my 6BR and even with the throat erosion they seem to hold their accuracy for a longer period (so far). I had been told by a Berger technician that the Hybrids like to be jumped and that they have a wide range of jump so less affected by throat erosion. One very successful F Class competitor/ gunsmith had told me that the 105 Hybrids work good in the .020" to .050 jump range and I have found that to be correct. With that I am thinking about trying the Berger 168 Hybrids to see if the same wide jump range holds true with them. Right now I shoot Sierra 168's and 175's in my .308 and they seem to have a good jump range. Berger does not make the 175 in the Hybrids. I have just started to check the erosion about every 300 rounds whereas I had only been doing it every 500 rounds and then resetting my bullet seating/jump accordingly.

 

[jlow]

This is what I was wondering i.e. even if a bullet likes the jump, whether one would have to adjust seating depth to account for the jump distance change. I mean even if they like to jump, it does not necessarily mean that all jump distance are good, just that the sweet spot in terms of seating depth does not have to be close to the lands or jammed?

The other thing is when one adjust jump distance, that of course affect case volume which makes me wonder how that affects the whole equation in terms of where the accuracy node/optimal jump distance is…

 

[Erik Cortina]

Accuracy will be affected in relation to how wide your seating depth node is and where you seat your bullets in relation to node. Some bullets are more tolerant than others.

 

[jlow]

That would make sense but honestly unlike a powder weight node, I have never actually notice much of a seating depth node width. My groups are usually reasonably tight by the time I do the seating depth adjustment having optimized via the OCW route and usually I find one depth that is tighter. On that note, it seems accuracy nodes are a relative thing. What I mean is when you find one say in OCW, the group size is not all the same across the node… So if something like throat erosion moves you around the node, even though you are still in the node, would your groups open up anyway?

 

[mikecr]

True seating depth testing(for best) is independent of OCW.
My conclusion with best seating so far is that best seating that is not ITL does not change with erosion -for the life of that barrel. That is, best seating holds as best. This also tells me accurate barrel life can be independent of seating.

But I can imagine that best seating that is ITL or jammed -to provide needed pressure, would be affected by erosion. This is really 3 changes at once, and I've never needed to manage it. I choose cartridges where it isn't needed.

 

[calgarycanada]

If you just change seating depth to keep same jump, won't it change the pressure curve? Quickload predicts that even 5 thou increase in jump can change pressure. Is that change enough to affect load tune?? Wouldn't you have to increase powder charge with seating depth??

 

[jlow]

Mikecr – yes, agree that seating depth testing is best done independent of OCW – that is what I do.

However, when erosion happens, the distance to the lands changes but of course seating depth remains the same (unless you change it). So this brings up the old question, which is when you optimize seating depth after OCW, what exactly are you doing? Specifically does that optimize involve the distance the bullet is to the lands, optimize the chamber volume, or a combination of both?

From what you say, it seems you experience is the distance the bullet to the lands is not the important factor but the resultant case volume is the determining factor. If this is true, then throat erosion should not change precision?

Yes, I am not talking about ITL/jamming.

Calgarycanada – what you ask is the same as my question in Reply #12

 

[mikecr]

Seating should not be tested after OCW. The first thing you should go for with a new barrel & bullet is a full blown seating test to determine best. Once you got it, you can do anything needed with powder & best seating will not change.

As far as seating OTL;
I have no idea why CBTO matters so much. My guess is pressure peak timing, not peak amplitude or powder burn contributing to muzzle pressure. And with this, powder charge changes do not change best seating. It remains. I don't think best seating is tied to case volume, or load density either.
I have no idea why original best seating holds through so much barrel life, but it does. If you find 10thou OTL is best to begin, you can go a nearly all, if not all of accurate barrel life, and full seating testing will show original CBTO still holding as best.
Maybe given bullet grip per given neck, applied to all else, to reach peak timing. PFM..

When I see someone chasing the lands, it seems to me like a barrel past it's accuracy peak, and not accepting it. Unless they're losing needed pressure node -with an ITL condition, I see it as tail chasing.
That is, by the time best CBTO changes due to erosion, readjustments are pretty precarious. They won't last anymore. IMO, a barrel setback would be more reliable.

But when a barrel leaves me, I leave it,, immediately. I can't stand to see it shoot marginally worse than it had been, & knowing it will continue to get worse faster.

 

[jlow]

Not sure there is a point debating which comes first in terms of OCW vs. seating depth. From what I have seen, seems like there are two different and opposite thoughts on this… In the end, if one can get an precise reproducible load, that is really what matters unless someone can come up with a reason why one is completely wrong.

Interesting thoughts as to the rationale as to what contributes to optimal seating depth, nothing to contribute to in that area but it is good to know that it does not change when the throat erodes – that is something that I did not know - thanks!