Reloading: Spike or Drop in velocity to determine max load?

[nincomp]

I have seen several recommendations that a good way to determine the max powder load for a particular bullet is to record the velocities and watch for a sudden spike or drop. Either sign will indicate the max safe load has just been exceeded.

My question is: Why?
It drives me nuts when I can't understand something like this.

Does this same pattern hold true for higher pressure cartridges like the .270 Win as well as for lower pressure ones like the 30-30?

Thanks

[LR1955]

I have seen several recommendations that a good way to determine the max powder load for a particular bullet is to record the velocities and watch for a sudden spike or drop. Either sign will indicate the max safe load has just been exceeded.

My question is: Why?
It drives me nuts when I can't understand something like this.

Does this same pattern hold true for higher pressure cartridges like the .270 Win as well as for lower pressure ones like the 30-30?

Thanks

Look at it in two ways.

First, do not exceed a recommended load with the Grendel if you are using a AR-15. Bolt guns are another story but don't do it with a AR.

Second, look at your powder charge in terms of efficiency. Say your max load from a reloading manual is 30 grains of X powder with Y bullet. You won't go over 30 grains due to safety issues but you will find if you chronograph your loads that as you increase your powder charge from what ever the minimum is listed in the loading manual that you will increase velocity. At first, the increases will be pretty big and as you increase more and more to the maximum safe charge, the increase in velocity will become less and less. Until you hit a point where you may see great variations in velocity.
We can tell using a chronograph because we will see a range of charges that show a very consistent velocity -- a low Standard Deviation and low extreme spread. So, you may see that at from say, 29 .4 to 29.8 grains of powder your velocities are not going up much but your SD and ES are very small. Given you are staying within a recommended load, you have probably found a charge that is as efficient as possible for your rifle and the bullet you are using. You know when you have gone over this range of charges because you will see spikes in velocities (ES) and the standard deviation will also go up. You will also notice that you are at or have exceeded a maximum load in reloading manuals.

As you increase your powder charge, you are also increasing your pressure but it doesn't level off. It keeps going up and when you start hitting max loads and above, it may go up much faster than during safe loads.

Yes, the same holds true for your .270 and any other cartridge. You may not know the pressures being produced but you can tell if they are consistent given velocity read outs and some statistics.

This doesn't mean the loads will shoot well. Only that you have found the most efficient load for that bullet and rifle.

LR1955

[LRRPF52]

As the pressure builds in a cartridge, smokeless powder kernels are transitioning from solid bodies, into hot gas very rapidly. The brass case acts as a high-pressure gasket between the building explosion, and your chamber. Certain charge weights with certain bullets and primers have predictable behavior.

If you exceed the parameters of predictability, this is where erratic pressure & velocities start showing up, which is bad, because they are unpredictable. What will happen? It usually is the case that the pressure spikes upwards up to the maximum tolerances of the cartridge and firearm until one of them gives. Nobody wants to be around when tens of thousands of PSI find another way to exit your firearm, other than the muzzle.

I see it when I push into the max published loads or when using a temp-sensitive powder in very high temperatures, and ES can be more than 100fps easily. It happened with my .260 Rem AR10 using N540 in a hot summer day, and I pierced primers. Maximum Average Operating Pressure for the .260 Rem is 60,000 psi, so who knows what pressure I was into. I know that my firing pin retaining pin was bent horribly from the force of the pressure finding a way back through the firing pin hole on the bolt face, and molten primer material had solidified in rings around my firing pin when I disassembled the rifle for cleaning later, totally not expecting to find damage and primer parts formed to my internal bolt geometry.

This is what is nice about the Grendel sticking to around 50,000 psi. With the 6.5mm pills, you get a lot of terminal efficiency for such a lower pressure cartridge with small case volume, very low recoil, and very low weapon weight. It's a balancing act, and I find myself shooting it more than my .260 Rem now, with my 5.56 blasters staying home as well.

[nincomp]

Last night, I was thinking about the apparent fact that when a chronograph indicates velocity excursions, the max load had been reached. Then a pertinent story occurred to me.:

Several people began to ask each other about the most impressive inventions of the past couple of centuries. Common answers were the airplane, the Apollo rocket and lunar lander, the telephone and the computer.
One fellow flatly stated that it was the thermos bottle.
"It keeps cold things cold and hot things hot."
"So?" his friends inquired.

"But how does it know?"

I reckon that my initial question could be restated as: "Max load has been reached when the chronograph starts giving excursions in velocity.
But how does it know?"