TAC isn't really one for reduced loads. I haven't tried it that way, but in normal load development in 223 and 308 it showed a preference for running near or at full throttle.
Exactly.
Like a lot of newer powders, TAC was designed to burn most consistently at pressures around 60,000 PSI.
When it was first introduced many handloaders didn't get this concept--but it's common with spherical powders, which aren't as "flexible" as extruded powders, due to requiring deterrent coatings to control the burn-rate. In extruded powders burn-rate is at least partially controlled by granule size: Larger granules burn slower, because they have less surface area. But sphericals have very similar granule sizes, so require coatings to change burn-rate.
Also, ALL spherical powders are double-based, meaning they have added nitroglycerin, which also affects burn-rate compared to single-based extruded powders. (Not all extruded powders are single-based, but many are.)
Consequently, spherical powders usually burn less consistently and completely in reduced loads. This was one of the early "problems" with TAC. When loaded to pressures around 60,000 PSI it burned VERY cleanly, which was unusual for spherical rifle powders at the time. But even with "starting" loads, say around 55,000 PSI, it didn't burn as consistently or as cleanly. Consequently quite a few handloaders tried it, and when starting loads left visible powder-fouling in the bore, and often didn't shoot very accurately, they didn't even try maximum loads.
Don't know how many handloaders contacted me about this, basically whining that it didn't burn cleanly or result in very good accuracy, but it was quite a few. I advised them to add more powder--and all reported fine results.
What all of this comes down to is that just because TAC has a similar burn-rate to IMR4895, it doesn't act anything like 4895 with reduced charges.