AusVarmint

Hi guys,

I did a load work up for my 7 Seddosguns (Lap Mag) the other day and the results are interesting when you put them in a graph.

NOTES:
Bolt lift started to get slightly harder at 99gr and at 103gr you could see slight ejector marks.

The nodes can be seen on the ES graph, 94-95gr and 99-101gr.

Are two rounds enough to get a reliable es? I'm interested not getting at you.

It's not ideal but it's enough to show a trend which is what I want. The next step will be to load up some ammo in the flat spots and shoot them for group.

Fair enough, I may give it a go

I hope there are more out there than me that just dont understand all this node stuff, reckon you could give a good explanation of just what makes a good node, are you looking for the best ..........?
And just how do you find these critters ?

Thanks

While I wouldn't normally put a huge amount of faith in 2 shot samples, your graph does show trend in ES climbing and shrinking again. Now the main deciding factor will be whether the velocity plateaus actually line up with any nodes. That is the real nodes, the barrel harmonic nodes. If your velocity plateaus line up with the barrel harmonic nodes, which are usually roughly 120-130 fps apart, then you are in luck, especially if they line up with the OBT (Optimum barrel time) nodes.
If you find that the barrel is still not accurate when shot at the velocity plateau speeds, you could likely move the harmonic node around to suit by putting on some small barrel weights of some sort, pretty much what a lot of us do with tuners.
Since you wont want to go firing long shot strings with a 7mm Lap, maybe a ladder test might tell you if you have actually found an accurate harmonic node. (it would be good if you could suck up to someone at the range with a labradar while you do the ladder test.

I agree that velocity plateaus don't always line up with real nodes but the times ive used it, it has happened more often than not, but not always at the higher node. I usually run the 2 shot groups while running in the barrel, it give you soem data while doing a boring job. I know the data can change once the barrel has been run in but its a good starting point. I also find out the limitation of the powder and where pressure starts.

A radar would make it easier than setting up a shoot through chrono but i dont know anyone who has one locally. I have been thinking of getting one.

A node describes the point where barrel vibrations are at the high or low point. When you shoot a round the barrel moves, like a small sine wave. The nodes at the top and bottom of the sine wave.

I thought that rather than the high and low points of the sine wave the nodes were the axis of symmetry of the sine wave, and that if the node corresponds with the muzzle then it doesn't deviate from that axis (being the centreline of the bore) and more accurate groups result.

Marcus

I think if you look at sine wave theory as proposed by Varmint Al and others it falls apart when you consider the vibration frequency of between 5,000-10,000 htz dependent on barrel length and profile. The likelyhood of a bullet exiting on an exact point of oscillation for successive shots is highly unlikely. Then there is also the question of whether the vibration is angular or wave.
Shock wave theory is far more plausible. If you look at Seddos data there are areas where ES closes up that would suggest that no gas is escaping through an expanded muzzle. Rather the muzzle has snapped shut at the 4 1/2 cycle as suggested by the theory and propellant gases are fully contained behind the bullet. It will be interesting to see if Seddos data matches up with accuracy.
As i have stated before, i have never seen a correlation between chronograph and accuracy. But Seddos data may prove me wrong. What is hidden is the primary vibration within the rifle in the order of a few hundred htz that many believe is where both compensation and ladder tests work in. This is the low speed vibration seen on super slomo video and is responsible for equipment failure and the vibration tamed at the muzzle through the use of dampers that Rod is eluding to.
https://www.google.com.au/url?sa=t&sour ... OOO5tJ71XD

Electronic targets were an eye opener for me, to see ammo that had a single figure ES at the muzzle ended up having 3-5 times more than that at 300y. I don't know if that had anything to do with the accuracy of equipment of not but it was not what i expected.

I read the OBT theory and part of the vibration theory a few years ago and came to the conclusion that it was more then i wanted to know, i just want to shoot and have fun. If my group is a little bigger at a certain distance i can live with it. I shoot with a couple of guys who shot groups at different distances with different jumps and charges and then they analyse them but that isnt for me either.

These days i'm happy to load a spread to see the working range of the powder and proije i want to use, i try some groups at the low ES spots and if they dont work i work my way back from the max load until i'm happy. With my 6mmBR i hit a happy spot around the mid 2800's and i stopped trying anymore loads, most of the guys tell me i should be 100+fps faster but i don't care.

I will shoot the groups on the flat spots and post the groups.

I find the concept of a barrel having a standing wave waveform somewhat unlikely.

Given the massive number of variables in play during detonation, burn, bullet travel, recoil, stock absorption of vibration, temperature delta, etc, etc, etc, I find it hard to believe that a standing wave (and therefore nodes and anti-nodes) could be set up. Especially as this is all happening in the few milliseconds before the bullet exits.

I can see a wave starting after the bullet has left and recoil has finished, assuming the barrel mounting system doesn't damp the vibration - but that is far too late with regards to accuracy.

Seddo wrote: ↑Tue Oct 03, 2017 8:58 am A node describes the point where barrel vibrations are at the high or low point. When you shoot a round the barrel moves, like a small sine wave. The nodes at the top and bottom of the sine wave.

Actually a node is a point of minimum amplitude. The top and bottom of the wave is the anti-node or point(s) of maximum amplitude.

In theory, if a standing wave exists, the node is the point of least movement.

I stand corrected