How to help Model a New Propellant
Author: Charlie
Users of GRT will notice that some commercially available propellants have no representative propellant model available from the GRT Propellant Database. That is because many of the manufacturers do not, or will not, share their internal data with GRT. GRT, however, has the ability to reverse engineer propellant model for use by the user, if the users will participate in the research required to start and build the models. The reverse engineering process requires averaging the measurements from users in the form of velocity, and/or pressure measurements the user gathers from well constructed firing tests. By well constructed we mean the submitted files have to be as precise as possible, and the velocity and pressure measurements are taken from instrumentation of known quality.
GRT is shipped with a basic sample data submission file that describes a minimum format for the submission of data for existing propellant model updating, but a start up submission is a little more complicated.
In order to create a new powder file in GRT you will need to run a 2x2x2x5 powder development run.
That is
- 2 calibers,
- with 2 bullets weights each caliber,
- with 2 powder charge weights each bullet weight (max and start),
- five shots.
That is 20 shots each caliber.
Shoot these loads over a good chronograph like a MagnetoSpeed or LabRadar. The chronographs like a ProChrono are not consistent enough for this use. Then you use GRT to make a .grtload file for each caliber (2 files), so 4 files. Name the files properly so it is easy to tell the caliber, bullet, and powder from the file name. It is most important that you measure the fired case interior volume in grains of water (brimfull level (ever so slightly convex) and enter that into the .grtload file. You also have to measure the length of your lot of bullets, and enter that as well, and the cartridge OAL. If the bullet chosen makes the OAL vary a bit, use the average length. See the pictures below.
Make sure you include your Load Data Source.
Also, include any notes that you think may be useful to GRT, put those is a separate Note Tab. The once you have the 4 files complete, just send them via the GRTLab-Plugin with a file name like “IMR 7828 SSC Powder Workup”. There should have been a Measurement file included in your GRT download, use that for reference. This hard work will start a powder file, and any other firing data submitted will help GRT to fine tune the model. Once GRT is satisfied the model looks viable he will send it to you, and with enough data he will add it to the next GRT release. Sounds like a lot of work, but GRT is free, so the user has to pay something if he wants something to work. Those users that are GRT Patreons also receive the newest propellant models more frequently than the free users. The frequency of model updates are directly related to the amount of data GRT receives.
The following .grtload files are samples of the type of data required in order to start a new propellant model. These are 2 calibers, with 2 bullets weights each caliber, with 2 powder charge weights each bullet weight (max and start), five shots. All users are free to download these files for inspection. The loads shown are not guaranteed to be suitable in any other firearm other than the one in which it was developed in. The 30-06 files include Pressure Trace II data in the Measurements Tab, and in a separate pictures/files tab. Storing auxiliary / external data files within a GRT Files tab makes it very easy to keep track of the data associated with your load workups. If the user has the Pressure Trace software loaded on his PC (available as a download from RSI), a double click on the tab's file name would launch the Pressure Trace software. It is also possible to display the pressure curves as an overlay onto the GRT Results Screen as shown below.
