Thread: Why plateaus in velocity graph

I've just done a 15 shot test with increasing increments of powder, looking for the fabled plateaus where SD is fairly low cross a couple powder weights. Question is, why does the plateau occur?

I BELIEVE its where the powder cant push projectile any faster ,either its run out of push or run out of barrel to push it in (flame thrower if go any higher)....

To date I haven't found any that couldn't be explained by statistical variation.

It might be a myth then Puffin?

Its true with black powder you could work out the max load by firing over a white sheet. You kept increasing the powder when you noticed black specks on the sheet you had exceeded max load. More wouldnt give you any more velocity but it wasnt dangerous it just didnt have time to burn before the bullet left the barrel.

The column on the lower left, is that the kind of thing you are looking for.

Yes, Cowboy that's what I mean. The theory that at certain ranges of powder charge velocity changes very little. Like this graph from the 6.5 Guys. They say we should focus on loads where the line is flattest since there, the SD is lowest. I don't know enough to dispute it but I can't get the idea to reconcile with anything I know about physics. Thought I'd ask if anyone else knew what was going on.

What is the correlation between the plateau of the velocity and the stabilisation of the barrel harmonics?

Can you have regular speed and shitty groups? And very good groups but very irregular speed?

Does "SD" refer to sectional density? Is that not just basically a formula for bullet weight over bullet frontal area, unrelated to velocity?

I'm guessing SD means standard deviation.

Could the plateaus be where the extra powder (and it's energy) are going into your brass instead of more velocity? As in the extra energy is stretching/deforming/working the brass more?

If you are shooting one shot at each powder weight, then repeat the exercise and see if it gives you the same or different results.

I’ve done it on heaps of rifles. Most of the time it works just fine. It gives you a pretty good idea of where to concentrate your 3 or 5 shot groups at the desired velocity depending on case pressure. You can also find your max pressure. Keep going till primers flatten and velocities usually peak then decrease 20 FPS. Adjust seating depth to increase accuracy. Not all nodes give good es. Or accuracy. Some people shoot 2 at each charge. You still get the same idea. It’s about speeding up simplifying and using less rounds for load development. I’ve found more often than not 1/2” or less single digit SD loads in 10-12 rounds. And tweak from there.

Thank you Cowboy06

Interesting observation. Never thought of why there would be flat spots in velocity, just that they are beneficial to identify and work with. I cant find any indication as to why on dr google.

If you think about it logically there is no way to gain any reliable indication of a plateau by using one shot per charge weight. The usual ES of most loads will make it completely random. @stug is on to it, if you think you have found something repeat the test twice more.