What determines the width of an accuracy node?

[fgregorio]

Hi all, from my limited experience (because I'm required to shoot only 155 gr in .308 and from a single competition rifle) I may help you with this.
I have done many ladder tests on the same rifle and this is what I came to:
1- I changed the stock from a wooden stock to a bedded alloy and it carried no difference on the sweet spot.
2- I honed the bore which pushed the lead 40 thou forward and it carried no difference on the sweet spot load, but it became slower, going from 2976 fps to 2910fps.
3- keeping the same jump, case weight and choice of primer, each different brand of projectile had different sweet spots, varying in width of the sweet spot and on what load.
4- change the jump and the sweet spot moves. Less jump, less powder or more jump more powder. But the width of the node remained the same.
5- I do ladder tests to 3 or 4 brands each time and choose the projectile that first, gives the widest node; second, that offers a velocity between 2950 fps and 3020 fps. Less is not good for 1000 yds and over. More than 3020 the recoil becomes too strong, which partially negates the quality of my hold. I shoot palma BTW...
Happy shooting
Fernando

 

[broncman]

Anyone tested the correlation of bullet bearing surface to bullet OAL length for node width?

Some bullets have a very long bearing surface compared to OAL . is there a link to that percentage and the width of the node or tunability?

 

[CharlieNC]

I know powder type can make a huge diffterence. When working up loads for my 6.5x47 at 200 yards, on the average the vertical POI increased 1.2" per grain of powder using RL15 while only 0.5" per grain for 2000MR. Likewise the nodes were <0.5 gr wide with RL15 and closer to 0.75 gr wide for 2000MR.

Both these had two nodes at the charges predicted using OBT via Quick Load, and I have determined this to be true for numerous other load developments. If you read Chris Long's article on OBT it is based on the pressure wave traveling along the barrel breech to muzzle (and back), not vibration. This cycle exhibits a much longer time period than the higher frequency harmonics associated with the barrel vibrating up/down, and I have never observed any POI behavior which appeared to be linked to the faster cycles.

So while it appears "straightforward" to calculate the location of the nodes, I haven't seen or determined a means to predict the width.

 

[seymour fish]

CharlieNC, Have back-tested the OBT theory using proven chronographed MV, and adjustments to Ba as necessary to match Quickload MV to proven, using loads that 'shoot', and have found that the Chris Long barrel times plus 1 to 2 % i.e. LONGER time, encompass almost all my good loads. outlier is an old 270 which likes 1% shorter barrel time than predicted. On the wide node deal, 300 win with n560=super narrow node; RL-22 node was near 2 grains wide. Good enough to pick the middle of it and go hunt across significant temp changes. Powder is a huge factor in node width, just may not turn out to be the one you 'wanted'. Seymour

 

[Brewz]

It is a combination of several factors. Harold Vaughn shows this in his book "Rifle Accuracy Facts". You can also look up Varmint Al's website and see the finite element analysis he did on barrels, actions and other things. It shows animations to help you understand. Barrel length, thickness, material, profile, action shape, the number of hole cut into it and their location all change the vibration patterns. The load and bullet are like selecting different mallets to strike a bell.

100% agree with this

I have found when working up a load with 2 different powders, one will shoot well at lower pressure, and one will shoot well at higher pressure.

I don't think there are any set variables you could list to determine accuracy nodes.

If you took 2 theoretically identical rifles and had them bedded by 2 different people, the accuracy nodes would be different.

I think every single aspect of the rifle contributes, hence why a load for me wont work perfectly for anyone else

 

[BCoates]

It is a combination of several factors. Harold Vaughn shows this in his book "Rifle Accuracy Facts". You can also look up Varmint Al's website and see the finite element analysis he did on barrels, actions and other things. It shows animations to help you understand. Barrel length, thickness, material, profile, action shape, the number of hole cut into it and their location all change the vibration patterns. The load and bullet are like selecting different mallets to strike a bell.

100% agree with this

I have found when working up a load with 2 different powders, one will shoot well at lower pressure, and one will shoot well at higher pressure.

I don't think there are any set variables you could list to determine accuracy nodes.

If you took 2 theoretically identical rifles and had them bedded by 2 different people, the accuracy nodes would be different.

I think every single aspect of the rifle contributes, hence why a load for me wont work perfectly for anyone else

How would you explain why factory ammo such as FGGM shoots pretty well in many guns? I haven't studied the subject of barrel harmonics as much as some people here, but I do believe that for a given carotridge size and bullet most of the good shooters find an accuracy node within a 50 fps range for a given barrel length.

 

[CharlieNC]

Brewz the vibration frequencies as calculated by Varmint Al are too fast to explain the location of the nodes according to Chris Long, and I observed the same on a very limited basis as well. That is not to say these vibrations may not important, but they do not explain the powder nodes because they are simply too fast. The OBT are based only on barrel length and rigidity does not play a role in this determination, but of course this is important. So matching OBT does not guarantee the rifle will shoot well, it just helps define the nodes where you can optimize the potential. Why does FGMM shoot so well? Plugging Dan Newbery's analysis of the load into Quick Load shows it meets the OBT criteria for a number of different barrel lengths.

 

[Brians356]

How would you explain why factory ammo such as FGGM shoots pretty well in many guns?

Depends on what your definition of "pretty well" is. For most people here, it's probably not nearly well enough.

Just saying that most folks applying the more elaborate, technical approaches to load development are trying to shoot into one ragged hole at 100 yards. A good "match" factory load might reasonably be expected to shoot significantly less than 1 MOA from most rifles, near 1/2 MOA from many, near 1/4 MOA from a lucky few. And that is indeed pretty good.

 

[BCoates]

I just don't think with the statement below. If both bedding jobs are good, the accuracy nodes should be in the same or similar places. One could confirm if there is a difference by having a barreled action bedded in two identical stocks, but why would you do that? My match gun is in a Manners stock with a mini-chassis, so how would that play into this. I would bet that I could take my barreled action and bolt it in another Manners T2A stock with a mini-chassis (identical to mine) and the results on target with my loads would be the same or within statistical error.

Brewz -"If you took 2 theoretically identical rifles and had them bedded by 2 different people, the accuracy nodes would be different."