As I’m doing more reloading I’m starting to get a real feel for how things work on the inside of a barrel. I’m not actually worried about the rest of the gun in this post, but rather internal ballistics of the cartridge interacting with the chamber and barrel.

The numbers I’m posting are all from QuickLoad, a simulation package that I’m using to get in the ballpark for what the loads I’m using are doing. The numbers match up with reloading manuals pretty well and I’m starting to build some trust in the results it’s providing.

Warning: The data presented here is not to be used as a reference for your own reloading. Please use a reputable reloading manual and know what you are doing. Don’t hold me responsible for if you blow yourself up. The loads I’m presenting here are well beyond the maximum pressure level as states by SAAMI and are for illustration purposes only!

The thing I’m trying to explore with this is how does pressure vary by amount of charge and the seating depth (really this is a measure of how much space the powder has in the case). Based on my previous post you will remember that powder is not a linear animal — the rate that it burns is related to the pressure/temperature it is subjected to. This is controlled by the amount of space it has to burn in — the seating depth and the bullet weight control those variables.

In all of these cases I’m using a .40 S&W case, Rainier Ballistics 165 grain bullets and Alliant Bullseye powder.

First off I’m going to vary the amount powder in the case (in this case is seating the bullet to a overall length of 1.130 inches):

You might wonder why velocity isn’t increasing at the same rate as pressure. There’s a simple explanation: The powder burns up quicker! Sure, you have a pulse of higher pressure, but then it gets used up. The key to accelerating a projectile is the area under the pressure curve (look further down at the graphs from QuickLoad). Just because the pressure goes higher doesn’t mean there’s a huge increase in the integral of the curve.

Next I’m going to vary the seating depth while holding the powder charge constant at 5.2 grains of Bullseye:

The thing that’s interesting in both of these charts is how quickly the pressure changes by altering just some of the variables.

Take seating depth for instance. When seated to 1.130 inches (what I’m using) the pressure is a tame 25271 PSI. The SAAMI spec for the cartridge is 35000PSI so it’s well under that. If you seat it just a tenth of an inch too short at 1.030 inces then the pressure skyrockets to 42326 PSI! That’s well out of spec for the design of the ammunition! It’s just a bit more than 20% over the maximum spec so things would likely survive, but that’s not what you should be going for. You could get the same result by not adequately crimping the bullet in and having inertia from other rounds seat the bullet deeper than you think.

So, what’s going on?

The powder starts to burn and the pressure starts to go up. The bullet starts to move, but it’s slow compared to the reaction of the burning powder. When you seat the bullet deeper, you give it less space for it to burn. As it burns the pressure goes up, which causes it to burn faster and so on.

Same thing with the powder charge varying — in this case though you’re changing the amount of energy being introduced to the case, but again the result is the same. Overpressure.

The two can interact of course, which is why experimenting with maximum loads is tricky. Going back the example before with 1.130 OAL and 5.2 grains of powder the maximum pressure is 25271 PSI and the muzzle velocity is 1033 fps. If you tweak things just a bit and seat the bullet just a 25 thousandth deeper  (1.105 OAL) and add 0.3 grains of powder the equation changes: 31711 PSI and a velocity of 1104 fps. It’s a lot closer to the danger zone. If you started off at maximum pressure, you’d be way over!

Ok, so that’s not dangerous — but you see what’s going on. Let’s go more extreme:

Let’s pic something that’s already a really hot load: the same 165 grain Rainier over 6 grains of Bullseye:

This one pushes the bullet to 1155 fps making 34375 PSI of pressure in the chamber. Remember the limit is 35000 for this.

Now, let’s push the bullet back by the thickness of a fingernail: 0.025″. (Yes, I went to the micrometer and measured my fingernails. They averaged to 0.021, but one of them was 0.0245″. Close enough. Your fingernails may vary.)

Oops! Now we’re up to 38511 PSI! Things will likely survive since it’s only around 10% over, but it’s hard on the gun and it’ll likely sting your hand as the slide smacks the frame. It’s generally bad mojo to do stuff like that in any case.

Excel of varying powder charge and seating depth

Remember — last time I promised you graphs!  :-)