In my experience down-loading aerodynamic models for spark photography in the .50 BMG, we used 4198 with good results, having no trouble with the case volume as long as the charge filled 50% or more of the available powder space. We used 110 grains of 4198 with the APIT M20 bullet to simulate the trajectory of the spotting round used for the old 106mm recoilless, but in the full sized 12.7x108mm case.
I agree that 3031 and slower powders do not tolerate airspace exceeding about 30%. I can tell you that 20 grains of Unique will drive the APIT M20 out the barrel of an M2HB, (if you oil the bore first!) ignite the tracer element, is "quiet" without a can, and shoots on at 200 yards with 1000 yard sight dope. It is a manual feeder, of course.