The burn-rate of TAC is very close to IMR4895/H4895, both of which have been used for reduced loads down to 60% of maximum charges for decades--though Hodgdon now recommends only H4895 be reduced that much, as they've found pressures/velocities become a little erratic in some cartridges with IMR4895.
But spherical powders are generally an entirely different deal. They're generally harder to ignite, and unless they fill the case at least 80% can be erratic, even with magnum primers. Compressed loads tend to work more consistently, no matter the primer.
Have used 42.0 grains of IMR4895 with bullets in the 400-grain range in my original 1884 trapdoor Springfield with excellent accuracy. Would not even consider trying the same charge of TAC.
