Multiple seating depth related accuracy nodes???

[dondlhmn]

I put this on the bolt action forum, too...but later figured I would get more input from the people on this forum:

Just kind of wondering about this after shooting my new 26" Krieger barreled 6.5 Creedmoor...It seems to me that I have found two accurate nodes...one with the 136L Lapua seated .010" off the lands and one with it seated .130" off the lands, with the tests in between those seeming to show that the .050 and .090 off not being all that great. I intend to single feed this rifle as it will pretty much only be used to develop the rounds and then to fire in a competition where the rules call for single feeding, so there is no real advantage to the ones seated in deeper to get the .130 off the lands. The .010 will not go through the magazine, but that is not really a factor for me, as whatever I settle on (or rather the rifle settles on) will be single feed only. So, it boils down to powder capacity, which seems, at least at this point in load development, to not be a factor as I tend to NOT load to the very maximum speeds I can get, and I am content with these 136 grain bullets heading out at 2875 fps or so, anyway. Also, pressures don't seem to be enough difference to point to one over the other....

So, ANYWAY...my question is: Have you all found there to be two pretty much equal nodes of accuracy which occur at quite widely varying seating depths? Is this common with a lot of rifles? If accuracy is related to barrel harmonics, it makes sense to me that accuracy nodes would occur at seating depths that are pretty far apart. What sort of experience have you all had with this?

Oh..yeah...ONE MORE...how much improvement have you all seen as a barrel "breaks in" when compared to how it does at, say, the 40 round count point? I know that different lengths, profiles and type of rifling all bear upon this, but it seems to me that a barrel improves for a while, then levels off for a loong time and then starts to go downhill when the end of the barrel's accuracy life is drawing near. Any experiences with this where you did enough testing and kept good enough notes to have formed an opinion? Maybe some manufacturer or barrel maker can chime in here?

I'd like to hear your comments and experiences, please. If you have anything to say about either of these questions, please PM me on this site.

Thanks!

 

[bruddah]

Tag for later.

 

[Exodus343]

Tag for later.

same here

 

[sermonator]

I'm don't shoot competition and guys that do might have better insight into this, but...

I've noticed that some bullets don't like "jump" (Berger). Therefore, they tend to like one length, which is going to vary depending on how the bullet interacts with the throat. Others bullets don't seem to be as length critical and still remain accurate despite "jump" (I've had good luck with Sierra regarding this). If the bullet is "jump tolerant" and engages the rifling in a consistent way, then it's all about the load and the barrel harmonics. Assuming the reloads are equally concentric in bullet, case, barrel relation, the accurate load is the one that exists the barrel at the same place and time as the barrel "bounces" up and down (the harmonic vibration). Imagine that the barrel harmonics have one load with the bullet leaving the barrel consistently at the "peak" of it upward vibration. Why couldn't their be an equally accurate load (but different) load where the bullet exists consistently at the "bottom" of it's vibration? So, while your experience might not be common, if the bullet is tolerant to jump, it might be that the accuracy of two loading depths isn't so much about the seating depth, but how long the bullet is in the barrel and when it exits. I'm not an expert on barrel harmonics, so maybe their are other factors involved, too. My point is that I've had some loads (in varmint calibers that I primarily shoot - .204, .223, .22-250) that seating depth and bullet jump didn't seem to have nearly as much to do with the accuracy of the load as is does in my large calibers (one 7mm RM, in particular).

Sorry, can't help you on the barrel question. I usually end up selling the gun and moving on to another before the accuracy starts to decline.

 

[bruddah]

I'm don't shoot competition and guys that do might have better insight into this, but...

I've noticed that some bullets don't like "jump" (Berger). Therefore, they tend to like one length, which is going to vary depending on how the bullet interacts with the throat. Others bullets don't seem to be as length critical and still remain accurate despite "jump" (I've had good luck with Sierra regarding this). If the bullet is "jump tolerant" and engages the rifling in a consistent way, then it's all about the load and the barrel harmonics. Assuming the reloads are equally concentric in bullet, case, barrel relation, the accurate load is the one that exists the barrel at the same place and time as the barrel "bounces" up and down (the harmonic vibration). Imagine that the barrel harmonics have one load with the bullet leaving the barrel consistently at the "peak" of it upward vibration. Why couldn't their be an equally accurate load (but different) load where the bullet exists consistently at the "bottom" of it's vibration? So, while your experience might not be common, if the bullet is tolerant to jump, it might be that the accuracy of two loading depths isn't so much about the seating depth, but how long the bullet is in the barrel and when it exits. I'm not an expert on barrel harmonics, so maybe their are other factors involved, too. My point is that I've had some loads (in varmint calibers that I primarily shoot - .204, .223, .22-250) that seating depth and bullet jump didn't seem to have nearly as much to do with the accuracy of the load as is does in my large calibers (one 7mm RM, in particular).

Sorry, can't help you on the barrel question. I usually end up selling the gun and moving on to another before the accuracy starts to decline.

My understanding is that the Bergers like to be jammed against the lands (or very small jump), or make a long jump (more than .100"). In between is bad from what I've read.

 

[rcamuglia]

Bergers Jump just fine.

Two accuracy depths in the .264WM at .040" off and .080" off

 

[88blazer]

I have a savage 12, 204. The stock heavy barrel from savage showed immediate signs of burnout when it happened. Then the Shilen barrel went on, still shooting great. The last couple trips out with the old barrel, I saw my 100yd groups open up to about 1.5", 50 more rounds, 2.5" group and a flyer or two 4" or so out. Accuracy in my case decreased really fast. I'd say probably throat erosion is what did it. This was my first barrel to burn out. 3 years of target and a ton of varmint hunting, I was happy with the life of savages barrel. But way happier with this shilen!

For jump, it was picky but not as much as bigger calibers I load for. Seating depth still made a difference. I was always way closer to the lands than book specs.