I have been using GRT for thick end of a year now. I really rate it. It has consistently given suitable loads for most of my center fires whose GRT predicted MV has subsequently been verified by chronograph on the range.
But there have been some exceptions. Principally some loads in 6.5x55mm have been off,
for me. The powder database appears to be a collective effort: user data improves accuracy. Helpfully, the powder database has a "calibration" guage indicating the depth of data on which the modelling stands.
View attachment 184984 Look at the stoplight graduated bar to the right of each powder.
One feature of GRT I stumbled upon recently was the Optimum Barrel Time [OBT] button. I surmise that the program is fine tuning the charge for maximum energy transfer to projectile for a given barrel length. What was uncanny is that the node I discovered for my .223 load by huge trial and error ladder testing is around the 24.8gr mar. The OBT pegs the optimum at 25.2gr. Had I seen that prediction, I would have had to build fewer ladder steps. I wonder if I need to add the moderator length...anyway there are gems in this freeware.
View attachment 184988