Max Pressure Uncertainty Error

rimfiremac

rimfiremac

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Surveying a couple reloading manuals has shown varying opinions for max load CUP (ie powder load) for the same weight jacketed bullet. In my case, it's for the 30-30 with a 170 gr jacketed bullet and IMR3031. IIRC, the Lee manual states around 28 gr of powder as max, while the Hogdon reference is up at 29.2 gr. Now for the same weight and style of bullet, I'd figure that the max powder load should be relatively constant, even across manufacturers (until you run into moly coating and such). A constant mass being accelerated over a constant distance to a specific velocity all relate directly to the driving pressure level and the friction while passing through the barrel- that's it. What gives?

Anyone have the equations for deriving CUP accurately btw?
 
Well - There are other variables - case volume, bore dia, jacket material, test temp, cartridge OAL, etc. The inertia force only accounts for a fraction of the driving pressure.
The point being is that max loads vary from situation to situation. I've had loads go relatively hot well below published max - thats why the manuals tell you to start at min load. Too many reloading newbies assume that max published loads are acceptable loads in their rifle - they're not, and it takes some skill to know where you stand.
There are commerial programs that develop pressure. Frankly, I think you'd be better off with a chrony.
 
Also, the manuals should list what barrel they used to develop the loads, different barrels take different pressures differently. I'd say start with the lowest min load, and work as high as the higher max load, if you're not getting pressure signs, remembering that pressure signs don't show up until you're a fair bit past safe. This is exactly the reason that it is generally adviseable to check at least 2 sources. You might get to the first lower max, and be at the top of your rifle's game, or you might only be half way there. Every rifle is different, only way to find out is to work up carefully.
 
The different manufacturers were like likely using different brass, primers, guns not to mention bullets and had different lawyers ;) . Some feel comfortable listing a higher load some dont'. I find the latest hornady varies quite a bit if you compare different cartridges....very wimpy in one and then downright retarded in another.

Like Dan said. You work up a load and you stop when you should or want. There have been a few times that I have unloaded that last few rounds because I no longer felt comfortable working up. Just goes to show that a listed max load is not an absolute in every gun its just data that was taken from the gun that they tested with.
 
Use the reloading manuals and work up from the starting loads and keep checking for pressure signs.

As for equations/softwere. There is only one good, relatively inexpensive internal ballistics program and that is QuickLoad. If you want the equations then download the trial version and look through the back of the Users Guide.

Software is actually very good but it takes a lot of knowledge to use it fully. Also, the software requires calibration with some of your own variables if you want to get the most out of it.

Most people just use the program as a type of "load book" without really understanding what is going on. (ie. studying/understanding the Appendices of the Users Guide)
 
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Considering different guns have different personalities, I can see how there's a certain amount of alchemy involved. Thanks for the insight- I think I'll try to track down Quickload and see what I can find.
 
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