04-30-2022, 01:21 PM
One thing I do to tweak Grt is the sebert factor. Some reading I did said this factor is about how much powder goes down the pipe as it burns... or something like that. Anyway the Grt help screens said you might tweak this to get some smaller cases to be more accurately depicted. The base value is 0.5 IIRC, I bumped it down to 0.45 for grendel and 6 Arc, and it **seems** to have helped. I also, after I get some actual chrono data on my load combos, I usually adjust case "volume" (more on this in a minute) so that MV closely agrees to the real MV. Then I look at pressure to see where it lands.
On the "volume" another debate is ongoing on the smaller fatter cases, that perhaps the primer igniting bumps the bullet out before all powder ignites... meaning there is an effectively "more" volume that the powder sees as its "case volume". So in some sense case volume might not always be the volume inside the case (for these smaller fatter ammo's), if you know what Imean.
Now these things I do are not patented nor proven to be what you ought to do; they are just what I have tried to make Grt a bit more useful to me. Most times, after doing my tweaks and comparing/adjusting to real MV data, I think of the resulting model as good mostly for a 1-off adjustment, like swap out a bullet, or change barrel length, change powder up or down a bit, or maybe (Gasp!!!) swap the powder type and see I get... The farther "away" from the precise individual combo-model you go, the more suspect and room for error you're probably going. None of which, of course, is quantified. So. Be careful out there.
On the "volume" another debate is ongoing on the smaller fatter cases, that perhaps the primer igniting bumps the bullet out before all powder ignites... meaning there is an effectively "more" volume that the powder sees as its "case volume". So in some sense case volume might not always be the volume inside the case (for these smaller fatter ammo's), if you know what Imean.
Now these things I do are not patented nor proven to be what you ought to do; they are just what I have tried to make Grt a bit more useful to me. Most times, after doing my tweaks and comparing/adjusting to real MV data, I think of the resulting model as good mostly for a 1-off adjustment, like swap out a bullet, or change barrel length, change powder up or down a bit, or maybe (Gasp!!!) swap the powder type and see I get... The farther "away" from the precise individual combo-model you go, the more suspect and room for error you're probably going. None of which, of course, is quantified. So. Be careful out there.