Bullet jump vs throat erosion

Greyfox

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Just a point. I have found that the single greatest effect influencing my getting accurate BTO readings is to be certain.that ALL carbon residues are completely removed from the throat area of a barrel that has been broken in when taking all measurements. I have seen errors as great as +.025” caused by Caron residue. Apologies if I’m saying the obvious.
 

Okanogan

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@Mikecr What got me started checking throat erosion vs jump was this article which seems to have progressed beyond the VLD jump question of old
https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/

@Greyfox no offensive taken by your point/ question. I'm not saying it is impossible for a carbon ring or something else to have affected the second set of reading, but I would have expected whatever it was to have affected all three bullets similarly. I didn't see any evidence of carbon ring when I scoped the bore yesterday but then I'm not an expert either. I did do an extra thorough cleaning job on the barrel though beforehand because I wanted to eliminate that as a potential cause of the unexpected pressure problems I experienced with RL26 yesterday (70 deg F).
 

Stiltsville

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This all interesting but when you normally set up monometals to jump 50 thou or more it's academic to hunting.

Once owned a 721 300 H&H with 2" of fried throat. Shot 3 TSXs into one inch seated to fit magazine. Single loaded with 180 SMKs seated to bite the throat it would put 5 in a Jefferson Quarter.
 

Mikecr

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Efforts in the article attempt to prove/disprove this sentiment: Benchrest World Champions and other experts explained that when seating bullets close to the lands, changing bullet jump by 0.002-0.005 inches can potentially have a dramatic affect precision.
If BR world champions was IBS 100-300yd, they're likely centered on 6PPC. There are 2 factors with that unrelated to what everybody else in the entire shooting world needs from seating.
#1 ITL seating is needed for high enough starting pressures to reach the extreme peak pressures of a competitive 6PPC.
#2 Tweaking of seating merely shapes grouping, but for competitive point blank BR, group shaping IS dramatic. Everything is dramatic.

The next matter I would mention about the article is that their focus was on pressure change sensitivities within 30thou of touching. Yes you affect pressure as you near touching because it raises starting pressure. But best seating is not about pressure or tuning. Best seated position is about resolving bullet-barrel interface issues otherwise.
Tuning changes from gradual raising/lowering pressures are best handled with actual neck tension adjustments (not what the mob thinks tension is), or adjusting kernels of powder.
Leave best tested seating right where best is.
This is the reason you should NEVER test for best seating from a tuned node. Those BR shooters are adjusting from a tuned node. Adjusting(tweaking), NOT full seating testing.
 

Okanogan

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@Mikecr I'm a just a hunter/ retired engineer, I'm definitely not a competitive shooter or any anyone claiming to be an expert. I was just happy to go back to the range today with a reduced load of RL26 and get back to +/- 1/2 MOA with the velocities that my 2B contour seems to like for the 156EH.

I'm kind of thinking that the lack of other data regarding how throat erosion changes jump for different bullet profiles may mean that there isn't a lot of data out there among our community. I get that as I have kind of shifted to focusing on the 156 EH because I am primarily interested in hunting with a precision rifle, I get why would others track changes for a other bullets they may not be focusing on? I will continue to put more rounds through the barrel and see what happens while continuing to check the affect on other bullets. If other folks have some data to share, I'd be most interested as I continue to be surprised by my initial results.

One thing I seemed to have found reinforced with my current load development effort, it is pointless to load up a bunch of loads in advance while your barrel is still changing. I had been up to just over 59 gr of RL 26 earlier before the start of pressure signs to now back to just under 57 grains with 100 fps more than when I started. Day one started with 57.0 grains under the 156 EH in the low 2800s and today I shot 56.8 grains at 2920.
 

Mikecr

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Are you sure the change in velocity is coming from your barrel? Or from changing brass attributes?
I know as cases change to close chamber clearances, pressure goes up. This, until it's settled.
I've never seen speed change in a barrel.
 

aushunter1

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From this link-
How Quickly Will the Lands Erode?

Short answer: “It depends.” But, I realize that isn’t helpful, so I’ll try to expand. First, it varies by cartridge, how hot of a load you’re running, and how hot you allow the chamber to get. If you fire long strings without stopping to let the chamber cool, it can erode much faster – maybe exponentially faster. In fact, it’s possible to burn out a barrel in one afternoon if you never let it cool (read article). It can even vary based on the barrel-to-barrel variation in steel, whether the barrel was button-rifled or cut-rifled, or the specific type of gun powder you use – so there are too many factors to make sweeping generalizations.

So even before they they put up their results there already saying there will be variables in any results!

I will throw this out there, if the erosion is due to the projectile then what effect would bullet coating have on this??

Secondly I would attribute erosion more to powder, types of powder & amount of powder.
We all know that overbore cartridges wear quicker because they are pushing all of those large amounts of powder at such a small diameter.
 

aushunter1

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Am I reading the OP's initial question & the title wrong then??
I do see he is talking about ogive shape in the mix but he is still referencing the PRB articles right
 

Okanogan

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I'm reasonably convinced the speed changes are coming from changes in the barrel. I have shot nothing but virgin ADG brass thus far and all the RL26 is coming from the same 8 lb canister and it was all shot with CCI 250 primers from the same brick. The temperature at the range has varied from probably 55-70F during my shooting over the last two weeks during the 250+ break-in rounds thus far. I use an Auto-trickler and a FX-120i for charge weighing. I have data that says the same load went up more than 50 fps from one day to the next during this period. I have cleaned the barrel extensively using more than one mfr solvent and have scoped it recently. Could it be something else, sure but it seems less probable to me than the new barrel break-in speed increase that others have suggested occurs.

I'm not disagreeing that anything is possible or claiming I know all the answers.

The rifle shoots well. My plan continues to be to try and gather additional data, including on other bullets I'm no longer pursuing for load development. Mostly I'm curious and haven't seen similar data from other sources.
 

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