Velocity indicating overpressure?

[phishfood]

Looking for an honest discussion here.

Reading this subforum, I have seen it said multiple times. That if you shoot a handload over a chronograph, increasing the powder charge slightly and incrementally. When you see the velocity either spike higher or drop lower, anotherwords, not follow the velocity increase that you would expect from the previous powder charge increases, that you have found the maximum safe pressure level for your barrel.

Not an extremely experienced reloader, so I don't feel like I have the knowledge base to call BS on this claim, AND that is not my intention with this thread. But I can't make sense of this. How does your chamber and barrel use excessive pressure to cause velocity fluctuations to tell you when to stop?

Consider that different caliber firearms can tolerate wildly different pressure levels. Pressures that a 5.56 AR can tolerate would distribute my .45 ACP across a 1/2 acre.

How is velocity spread an semi accurate indication of safe pressure?

[centerfire]

Looking for an honest discussion here.

Reading this subforum, I have seen it said multiple times. That if you shoot a handload over a chronograph, increasing the powder charge slightly and incrementally. When you see the velocity either spike higher or drop lower, anotherwords, not follow the velocity increase that you would expect from the previous powder charge increases, that you have found the maximum safe pressure level for your barrel.

Not an extremely experienced reloader, so I don't feel like I have the knowledge base to call BS on this claim, and that is not my intention with this thread. But I can't make sense of this. How does your chamber and barrel use excessive pressure to cause velocity fluctuations to tell you when to stop?

Consider that different caliber firearms can tolerate wildly different pressure levels. Pressures that a 5.56 AR can tolerate would distribute my .45 ACP across a 1/2 acre.

How is velocity spread an semi accurate indication of safe pressure?

Velocity spikes most likely would indicate pressure climbing to unsafe levels but that depends heavily on the type of powder used. Most of the powders I have chronographed show linear velocity increases with incremental powder charge weight increases. I think the most common exception to my testing, and one you will see discussed frequently, is 8208 XBR which tends to spikes at the upper end of pressure.

[Labrat198]

This is something that I have been trying to figure out for myself as well, without much luck I might add. I have been fairly diligent to stay within published data or at very least have some point of reference I feel is reasonably similar. I have mainly been using XBR and H335, sometimes there will be a plateau where my velocity will level off or even dip slightly before continuing to climb at a steady rate. Every time I have had a large spike or drop I have been able to reasonably attribute it to the sun causing reading errors. Each time I have gone back another day, working up to the loads that I had issues with and everything will be as expected.

[Klem]

Phishfood,

I agree with you. I don't believe steeper changes in the relationship between velocity and powder are necessarily indicators of crossing an arbitrary safe pressure threshold. It just means you have to load incrementally more carefully and your results on the target might not be as precise.

Apart from having to be more careful when we see changes in proportionality between powder and velocity I don't believe 'spiking' is an indicator of 'over-pressure'. Over-pressure meaning when the pressure crosses an arbitrary safe-working-load threshold, like SAAMI. To be honest the only time I have seen this happen is when the available case volume becomes full and it turns into a compressed load situation. I know this because I fill a couple of empty cases with water and weigh them. Then enter this into Quickload along with the bullet, the OAL and amount of power. It indicates the percentage of available case being filled with powder. Once it reaches 100% this is when I notice the velocity/powder line gets steeper. It goes from directly proportional (straight line) to logarithmic (curved and steeper, or spiking). At about 103% is when I start to feel loading press resistance and the crunching of kernels. For those who don't use an internal ballistics program or have this information from books and tables this would be the first time they sense that powder is being compressed. But prior to this is when the case becomes full and the spike starts. No doubt the compression of powder is changing the burning rate which along with decreasing boiler room space and more powder are all contributing to this steeper increase in pressure/velocity.

With 8208 it is a faster powder than others we typically use and is at, or around SAAMI limits at a lot of the typical loads on this forum (according to QL). It is also typically at 100% available case capacity at COAL of around 57.5 - 58mm (the internal length of an AR magazine). This means it is an efficient powder for this calibre but it also means pressure/velocity will quickly spike if you try and put any more powder into it. There's nothing unusual about 8208 compared to other single based powders, but it's because it is so efficient for the Grendel case volume that also makes it potentially dangerous.

I use 8208 for bullets up to 120gn.

[Jimla]