Originally Posted by steve smith
Competition Electronics ProChrono
I know it's not the best, but it has been farely reliable, so long as everything is square and level.
I played with the H2O capacity, but didn't think of adjusting the OAL!
But! If you're only getting within 1-2% then I'm probably close enough.
Guess I'm just being a little too anal. That's what I hate about Engineering Data, it's a very good approximation, but for someone like me (having a machining/fabrication background) being off 1-2% seems like a mile. It's a wonder, why I'm in school to become an engineer! I'm having to learn when to say "close enough".
I've found the following technique to work for me......
Go to range with 10-15 different powder charges using the same bullet/loading methodolgy (neck tension, primer, case prep, etc). For example....from the published starting load to the published max load for your case/powder/bullet combination in one grain increments it spans 12 grains. 83 grain starting load, and it ends at 95 grains. You'd load one bullet at each increasing powder charge. Record velocities from shot string.
Then in QL, "build" the cartridge for your gun by
1. Input actual water case capacity
2. Input correct COAL
3. Select powder
4. Select temperature
5. Input barrel length
Once the cartridge is set, I adjust the case weighing factor and the start pressure in attempt to match my gun to the lowest charge velocity, and the highest charge velocity. I've found if you can get QL to model the 1st shot velocity and the last shot, it will predict the "steps" between the two on the increasing powder charges. (Remember to throw out your first few fouling shots as they will throw the velocity step/spread off)
The reason for using the published data, is some reloading references list the specific pressure for a given velocity, and barrel length. Now every gun/throat/barrel is different, but that's an additional factor of analysis that you can compare with your gun. Example: your gun is a 26" barrel just as is listed in the published loading data. You achieve the max load velocity, but with two grains less than the max load. To assist in achieving an accurate model, try adjusting the case weighing factor/start pressure until QL achieves the same max pressure listed in published loading data for that specific velocity. After doing that, check the rest of the "steps" on the incremental loads and see if the velocities are matching up.
If you have a different barrel length, be sure to correct for that when comparing published velocities/loads/pressures.
Once you have the case weighing factor/start pressure down record it for future use. For example, I found with a 300 RUM 40 degree shoulder that .35 on the case weighing factor works well, and for .025 from lands, a start pressure of 8000 works in one gun of mine.
Now using that same case weighing factor and start pressure, QL does a very accurate prediction of how other powders will perform.
The start pressure doesn't seem to be a hard and fast rule, and seems to difficult to predict when changing neck tension or distance from lands.