Been my experience that the load data listings for TAC are way off. I had some older powder laying around, and then some fresh powder and noticed a difference so I immediately switched to the new batch since I had plenty of the new stuff and to save the effort. I didn't bother with investigating why there was such a big difference between old and new or manuals and math models.
I cannot be responsible without warning you that it is never advisable to contradict the published manuals without a good understanding that internal ballistics can be very dangerous when you don't understand safety margins and safe practices.
This post will only show the results of two runs of GRT to illustrate a point, which is that M193 velocity has a history of being misunderstood by many folks due to several factors.
The specifications were written long ago when many facilities had a type of chrono system that did not measure at the muzzle. As a result, those measurement systems were written into the specs. Also, many published manuals fail to point out the differences in test barrels and pressure standards they are publishing, and importantly at what length.
I will show a pair of runs with the Hornady 55 FMJBT bullet using 55 kpsi as a max pressure level, but from two different barrel lengths.
To stay at or just below 55 kpsi requires a max of 27.9 grains, but I am not recommending you do this. I am just illustrating the rough velocity drop and also how different an internal ballistics tool like GRT can be from published manuals.
At 54.4 kpsi, with a 20" bbl typical of a service rifle, your velocity at 27.9 grains (98.2% fill) would be roughly 3200 fps.
But, if you bring that same set of input variables with a 14.5" bbl, the result is a muzzle velocity of only 2900 fps
You may find your bbl follows a little higher or lower performance, but you would still see a roughly equivalent speed change just based on your 14.5" bbl versus what a 20" service rifle or 24" test bbl will produce. (3184fps - 2900 fps)/ (20" - 14.5) = roughly 50 fps per inch in speed drop. Your bbl is 5.5" shorter, so you should expect to see roughly a drop on the order of 275 fps.
ETA: I got pulled away so I cut it short. Will add that I would recommend you don't assume matching a velocity is going to match precision or function when using different powder (or other parts of the recipe for that matter), but if I were trying to clone loaded ammo by reloading, just using the rough velocity and adjusting up or down a little is worth the effort. YMMV
Merry Christmas, Happy Hanukkah and Happy New Year!
