Bullet jump vs throat erosion

[Okanogan]

There has been a considerable amount of discussion recently since the PRB series about throat erosion and jump. I've been working up load for a new GAP 6.5 SAUM and for the most part enjoying the journey trying out some of the PRB ideas. I have had mixed success with groups ranging from 0.3 MOA to 0.5 MOA for what I considered the "good" loads. (and of course I have had much less successful loads along the way). I have about 225 rounds through the new barrel and so consider it more or less broken in.

Before I fired any rounds through the new rifle, I stripped the bolt and used the Wheeler method to set up reference cartridges for bullets that I was interested in testing out with the new rifle. Today I stripped the bolt again and repeated the exercise of determining distance to the lands for several bullets. Here are my results:
156 gr Berger EH initial COAL 2.861 to lands/ current COAL 2.929 to lands
140 gr Berger EH initial COAL 2.956 to lands/ current COAL 2.958 to lands
147 Hornady ELDM COAL 2.774 to lands / current COAL 2.801 to lands

I know a lot of folks think COAL sucks vs measuring to the ogive, but these are the same bullets in the same original reference cartridges. My initial thought was I must have really screwed up the Wheeler test for the 156 EH but I will say in my opinion there is no way you could screw it up by more than 0.060 and not notice, that and 156 EH are the best shooting bullets in the new rifle.

My current hypothesis is I think some bullet shapes are more inherently susceptible to changes in throat erosion in terms of bullet jump than others because of their geometry. (i.e. a really tapered bullet may be more affected than a less tapered bullet) I would have considered both the Berger bullets above to be pretty tapered so this theory might be worthless as the change in COAL are really different. I know it is possible that I blew the COAL for the reference round on the 156 EH but that still wouldn't really seem to explain the relatively small change for the 140 Berger EH vs the ELDM which is not nearly as tapered as the Bergers.

The PRB articles on bullet jump and throat erosion were pretty interesting to me because I really had never consider throat erosion very much in my reloading practices. I'm sure some bullets are more forgiving than others on jump, but I wonder if some bullets are inherently more tolerant/ susceptible to throat erosion than others. I've seen some pretty insightful comments recently from Alex Wheeler, et al. I'm interested in feedback on the above information and what it might all mean in the context of the PRB articles.

 

[YZ-80]

I don't know what to say about this one. It is an interesting thought. I'm guessing that ogive profile (secant vs. tangent) could theoretically have an effect. I've been loading Nosler 115 BTs in my Cooper .25-06 for 10 years, and I still am. I've lost 34 thousandths off the throat and I'm chasing it but the gun remains .5 MOA 10 years later. It would seem that a longer "jump" would impact the lands with more force at a given charge and maybe accelerate erosion. That said, as erosion occurs, maybe switching to a different ogive profile (fatter, perhaps) would result in better engagement of the rifling as things wear. I don't know. But it's an interesting thing to ponder as I wait for varmints on this beautiful Labor Day weekend evening.

 

[26Reload]

Good test...i also am using that method for measuring...
I wonder if in the case of bullet jump...that the bullet having the extra room to move before hitting the lands..gives it that extra power to engrave into the lands....

Let's say the load is anemic...faulters in ignition and imprints the load into the lands but fails to travel the barrel...would that bullet still be able to grip into the lands...or would the bullet be squashed into the barrel not filling the grooves.....

So.... theoretically...the more burst of speed to hit the lands should would have less impact of distorting the barrel metal?

I shoot nosler lrab's....and they have a softer front end....

 

[Tiny Tim]

I had a puzzling experience with a rifle and a couple of bullet combinations that defied my understanding and someone here helped me out. I would have expected the two Elite Hunters to have similar changes in jump due to similar profiles. A couple questions, Did you do this initial testing on a new unfired barrel and in this order? Perhaps a burr or chip from chambering caused an error on your first measurement on the 156 EH. Was this your first attempt at this method and your getting better/more consistent in your technique. Different bullet profiles will engage the leade at different points and as the throat and leade erode, the shape of that engagement surface may change some, causing various results, but with such a large variation on one and not on the other of similar profiles seems to point to a measuring error, no matter the cause.

 

[DJ Fergus]

There has been a considerable amount of discussion recently since the PRB series about throat erosion and jump. I've been working up load for a new GAP 6.5 SAUM and for the most part enjoying the journey trying out some of the PRB ideas. I have had mixed success with groups ranging from 0.3 MOA to 0.5 MOA for what I considered the "good" loads. (and of course I have had much less successful loads along the way). I have about 225 rounds through the new barrel and so consider it more or less broken in.

Before I fired any rounds through the new rifle, I stripped the bolt and used the Wheeler method to set up reference cartridges for bullets that I was interested in testing out with the new rifle. Today I stripped the bolt again and repeated the exercise of determining distance to the lands for several bullets. Here are my results:
156 gr Berger EH initial COAL 2.861 to lands/ current COAL 2.929 to lands
140 gr Berger EH initial COAL 2.956 to lands/ current COAL 2.958 to lands
147 Hornady ELDM COAL 2.774 to lands / current COAL 2.801 to lands

I know a lot of folks think COAL sucks vs measuring to the ogive, but these are the same bullets in the same original reference cartridges. My initial thought was I must have really screwed up the Wheeler test for the 156 EH but I will say in my opinion there is no way you could screw it up by more than 0.060 and not notice, that and 156 EH are the best shooting bullets in the new rifle.

My current hypothesis is I think some bullet shapes are more inherently susceptible to changes in throat erosion in terms of bullet jump than others because of their geometry. (i.e. a really tapered bullet may be more affected than a less tapered bullet) I would have considered both the Berger bullets above to be pretty tapered so this theory might be worthless as the change in COAL are really different. I know it is possible that I blew the COAL for the reference round on the 156 EH but that still wouldn't really seem to explain the relatively small change for the 140 Berger EH vs the ELDM which is not nearly as tapered as the Bergers.

The PRB articles on bullet jump and throat erosion were pretty interesting to me because I really had never consider throat erosion very much in my reloading practices. I'm sure some bullets are more forgiving than others on jump, but I wonder if some bullets are inherently more tolerant/ susceptible to throat erosion than others. I've seen some pretty insightful comments recently from Alex Wheeler, et al. I'm interested in feedback on the above information and what it might all mean in the context of the PRB articles.

Glad you started this thread, I have load work for a 6.5 gap coming up soon. I will be trying the bullets you mentioned. I also have been reading some of the article about bullet jump & throat erosion tolerance. Please let me know what your findings are. I have found good seating depth with eldm bullets at .047 from lands in more than one rifle.

 

[Okanogan]

The initial measurements were all taken before the barrel was shot for the first time. I was surprised at the initial measurements and the significant differences for each bullet. I'm pretty confident that the 156 EH initially had significantly shorter COAL to the lands than the 140 EH. I was kind of disappointed because I wanted to be able to seat the heavier bullet closer to mag length like the 140 EH. Could there have been some kind of initial imperfection in the new bore- sure but I would have thought the impact on measurement would have been roughly similar on all the bullets. The recent measurements taken for comparison where all taken at the same time yesterday.

In general when I am using the Wheeler method, I go at a fairly coarse 0.010 inch seating increment and then go back and pull the bullet and repeat at 0.002-0.003 increments. This speeds things up and kind of acts as a double check as well to avoid gross errors. I'm not saying it is impossible that I screwed up a measurement but I've been using the Wheeler method for some time and have found it to be pretty repeatable. Unlike the Hornady overall length gauge that I tested out, I think missing the COAL by 0.020-0.030 with the Wheeler method would be unlikely.

I am very surprised by the results I got. I was expecting all three incremental change values to be pretty close. I was hoping others might have similar info they could share or if not perform similar checks in the future to see if my results are an anomaly. My own next steps are to reload some 156 EH using the new and old COAL info and go out and shoot some more. When I get another couple hundred rounds through it, I'll check again to see how things have changed.

@djfergus I have been having better luck with Bergers than ELDM/ELDX in my load development. The GAP reamer vs profile of the Hornady bullets requires me to seat them much deeper than the Bergers as you can tell from the initial post. You may have better luck with the ELDMs than I did but I wouldn't recommend stocking up until you find out.

 

[DJ Fergus]

The initial measurements were all taken before the barrel was shot for the first time. I was surprised at the initial measurements and the significant differences for each bullet. I'm pretty confident that the 156 EH initially had significantly shorter COAL to the lands than the 140 EH. I was kind of disappointed because I wanted to be able to seat the heavier bullet closer to mag length like the 140 EH. Could there have been some kind of initial imperfection in the new bore- sure but I would have thought the impact on measurement would have been roughly similar on all the bullets. The recent measurements taken for comparison where all taken at the same time yesterday.

In general when I am using the Wheeler method, I go at a fairly coarse 0.010 inch seating increment and then go back and pull the bullet and repeat at 0.002-0.003 increments. This speeds things up and kind of acts as a double check as well to avoid gross errors. I'm not saying it is impossible that I screwed up a measurement but I've been using the Wheeler method for some time and have found it to be pretty repeatable. Unlike the Hornady overall length gauge that I tested out, I think missing the COAL by 0.020-0.030 with the Wheeler method would be unlikely.

I am very surprised by the results I got. I was expecting all three incremental change values to be pretty close. I was hoping others might have similar info they could share or if not perform similar checks in the future to see if my results are an anomaly. My own next steps are to reload some 156 EH using the new and old COAL info and go out and shoot some more. When I get another couple hundred rounds through it, I'll check again to see how things have changed.

@djfergus I have been having better luck with Bergers than ELDM/ELDX in my load development. The GAP reamer vs profile of the Hornady bullets requires me to seat them much deeper than the Bergers as you can tell from the initial post. You may have better luck with the ELDMs than I did but I wouldn't recommend stocking up until you find out.

Thank you for the heads up. I have 147 elds but I guess I will go ahead and order some 156 Berger's. I haven't shot or even put the barrel on yet cause I sent it out for fluting. I did bore scope the chamber, throat & barrel. It is a .298 neck .81 free bore. The throat looked very different to me than my other throats. It almost looks like what I would think is a bore rider throat as the lead into the lands is very close to the neck but angles farther into the bore than throats on my other rifles. IDK, maybe it's normal. Have you looked at the throat on yours? Does this seem like anything you've noticed? James Skinner of Apache gun works Chambered it and he's done good work for me before.

 

[Mikecr]

You could normalize these measurements by taking them to CBTO.
With this, you would see that the attribute changing in your barrel is leade angle.
What I have found over the years is that initial tested best CBTO (while off the lands OTL) holds, regardless of throat erosion. In other words, if initial tested best CBTO is OTL, you can log that, and never need to change it for the life of that barrel -with that bullet. NO NEED TO CHASE LANDS

Make the choices that mitigate any need to chase lands, or accept fleeting accuracy.

 

[Okanogan]

@djfergus doesn't sound like you are using the GAP reamer so your results may very from mine. I did run my low cost Teslong borescope through the rifle today. I didn't notice anything jump out at me but I'm not a gunsmith either. I ran the bore scope because todays range session was more or less a wash out. RL26 powder charges that gave me less than 1/2 MOA a week ago with 156 EH where over pressuring. (@57.9 gr) The velocities were up bouncing back and forth above 3000 fps which is where I start showing pressure signs with the 156 EH. Also in my 2B contour the groups start opening up above about 2950 fps. I thought maybe a carbon ring or chamber fouling might be the cause as I thought I was past things speeding up. A week ago it took 59+ grain RL 26 to break 3000 fps and 57.8-58.0 put everything in the same hole at about 2940.

I would definitely suggest giving the 156 EH a try, especially if you have RL26 or something similar. I struggled to get them above 2900 with H1000 and my barrel harmonics seem to improve just above that. I would expect yours to be different but who knows. The 156 EH are the best shooting bullet I have tried so for in this rifle, which is probably what I would wish for as I want a hunting rifle for this rig.

@Mickecr I get that the original best shooting jump may retain its advantage. That was part of my testing plan today, but it kind of got scraped when velocities and pressures continued to climb. Because I am using the same original bullet and cartridge as a reference point, I think the difference between CBTO and CAOL will have less of an impact on my testing, especially because my original preferred jump was 0.060 which seemed to be a little more forgiving. My plan had been to shoot some of the original preferred COAL along with the newer loads but that will have to wait for the next session with less powder.

I'm still courious if others have data that says different bullets' jump change the same as the throat erodes.

 

[DJ Fergus]

@djfergus doesn't sound like you are using the GAP reamer so your results may very from mine. I did run my low cost Teslong borescope through the rifle today. I didn't notice anything jump out at me but I'm not a gunsmith either. I ran the bore scope because todays range session was more or less a wash out. RL26 powder charges that gave me less than 1/2 MOA a week ago with 156 EH where over pressuring. (@57.9 gr) The velocities were up bouncing back and forth above 3000 fps which is where I start showing pressure signs with the 156 EH. Also in my 2B contour the groups start opening up above about 2950 fps. I thought maybe a carbon ring or chamber fouling might be the cause as I thought I was past things speeding up. A week ago it took 59+ grain RL 26 to break 3000 fps and 57.8-58.0 put everything in the same hole at about 2940.

I would definitely suggest giving the 156 EH a try, especially if you have RL26 or something similar. I struggled to get them above 2900 with H1000 and my barrel harmonics seem to improve just above that. I would expect yours to be different but who knows. The 156 EH are the best shooting bullet I have tried so for in this rifle, which is probably what I would wish for as I want a hunting rifle for this rig.

@Mickecr I get that the original best shooting jump may retain its advantage. That was part of my testing plan today, but it kind of got scraped when velocities and pressures continued to climb. Because I am using the same original bullet and cartridge as a reference point, I think the difference between CBTO and CAOL will have less of an impact on my testing, especially because my original preferred jump was 0.060 which seemed to be a little more forgiving. My plan had been to shoot some of the original preferred COAL along with the newer loads but that will have to wait for the next session with less powder.

I'm still courious if others have data that says different bullets' jump change the same as the throat erodes.

I've got RL26, retumbo, 3lbs of h1000, & n570. I really like RL26 but when temps get up around 85 degrees or more, I have saw it do what you are talking about. I'm not saying that's your particular issue but it's just something to think about. When it's 30-70 degrees I don't find RL26 to be finicky.
I would like to try n570 but I honestly think it can sometimes give problematic ES. Its fine in my 7mm stw but I have found it to be a little finicky in some other cartridges.

 

[Mikecr]

Keep in mind that your ogive shape/land relationship notion in question was outright pooch poked for decades about VLDs. All the 'experts' were wrong..
Truly, the important matter in this is FULL seating testing to begin. I hope you realize that best is usually different for different bullets.
I think you'll be fine

 

[DJ Fergus]

Keep in mind that your ogive shape/land relationship notion in question was outright pooch poked for decades about VLDs. All the 'experts' were wrong..
Truly, the important matter in this is FULL seating testing to begin. I hope you realize that best is usually different for different bullets.
I think you'll be fine

I'm not doubting you. I guess I'm a little uninformed or unsure what you are referencing. Are you talking about the matter of jumping the original vld when they had declared it needed to be jammed then figured out later on that it could be jumped and jumped a good bit and still shoot with good accuracy?

 

[Mikecr]

Yes that's right. It was embarrassing really.
The mob rolled right over those that had seen & knew otherwise.

 

[DJ Fergus]

Along those same times, I was jumping a 162 sst around .080" with good accuracy. It had a secant ogive just as the original vlds.