How accurate is the simulation? How are the variations rated?

General Rule

GRT is after all a simulation. Expecting a simulation to be spot on, or at least 99.5% accurate without calibration1), in every instance, is to be discouraged, and not realistic. If the user fails to verify all of the parameters for accuracy, and just relies on accepting the default values of the provided databases2), a poor result is likely.

Rifle Cartridges

For rifle cartridges, in general, a deviation prior to calibration3) of e.g. 3% or less, such a simulation would be rated as “excellent”. For up to 5% would be “very good”, and below 10% as “good/satisfactory”. Even though the variation is often less than 3% with a highly rated powder model, quality components and good loading practices, GRT is not a Magic Ball!

Pistol Cartridges

For pistol cartridges the story is somewhat different. Since pistol cartridges have much less powder charge in comparison to rifle cartridges, the available energy is much smaller. Therefore, all of the component variations/influences have a much higher impact on the deviation/error rate. With pistol cartridges the ratings are 6% or lower as “excellent”, up to 10% would be “very good” and below 20% as “good/satisfactory”.


GRT is able to predict rifle cartridges often below 3%. That is amazing enough when you look at the possible influencing factors and their variations (given reasonably good components):

  • Powder lot-to-lot variation approx. 1-2%+
  • Case volume approx. 1.5%+
  • Reloading accuracy / tool variations 1.5%+
  • Bullet length up to 3%
  • Bullet mass approx. 1%+
  • Special bullet constructions or materials: 1-10%+
  • Environmental conditions such as temperature, humidity, etc.: up to 1-5%
  • Barrel/weapon wear 1-2%+
  • Chronometer accuracy: up to 5%
  • Primer type, construction and components: 1-2%
  • Physical effects such as projectile protrusion by primer before full powder ignition (mainly with cylinder cases or large calibers): up to 10%
  • etc.

If you now add up several of these variations in the worst case scenario, you know why it is recommended to start charge development at -25% Pmax or below.

Hopefully this explanation gives you a perspective on the simulation possibilities and how to rate the limits and capabilities of GRT's simulation for yourself. That a simulation can be predominantly below 5% remains a mystery that amazes us as well. It is important to recognize that these variations do exist in reloading, the components/tools used, how to minimize them and how to best use the (additional) tool “Simulation”.

1) , 3)
The word “calibration” refers to the process of adjusting the simulation Initial Pressure (IP) parameter, or a particular propellant models' Ba and k values using the GRT OBT-Tool in order to match the simulations velocity to an actual test firing of the subject loading.
e.g. like fired case volume and bullet length

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  • en/faq/faq-simaccuracy.txt
  • Last modified: 2021/05/30 03:34
  • (external edit)