to make up a couple of dummy rounds for a custom chamber in 375 Flanged Magnum. I will probably shoot the Hornady 270gr SP at deer and dirt clods most of the time. If the chamber is made using dummy rounds with this Hornady bullet, do you think I would be able to also load a 300 gr flatpoint?

I would use the 300 gr. Sierra for a dummy, then most any bullet will fit your gun,,,,I wouldn't worry about bullet jump with a short bullet as it won't effect that caliber and that type of gun...If it were my gun then I would just use a fairly long throat like any double and shoot whatever bullet I wanted or whatever it would shoot.

I agree with Ray -- my .375 Whitworth has such a long throat that at least the lighter bullets would be totally out of the case to touch the lands. But it shoots 1.5" or less with any load, as long as the bullets are seated straight.

John