The rifle that got my father - and subsequently me - into casting is a replica of the Civil War era British / Confederate Whitworth sniper rifle. In the event that you're unfamiliar with the gun and its rather unique projectile, here's a good link that lays it all out:
In a nutshell, the round-nose bullet is a .451 (across the flats) x .459 (across the points) hexagon with the flats slanted to mechanically fit the 1-20" (possibly 1-22") twist of the hexagonally-profiled bore. The mold we are using makes a slight hollow base for inserting the "pigtail" of the paper patching these bullets are typically wrapped in to prevent fouling.
Here's a link I found with pics of the mold we're using that also shows bullets produced in same:
The downside is that these long, hollow-based, nose-poured, punched-out-by-the-nose bullets are a BEEE-YOTCH to cast cleanly. It seems like that bullet design is an ideal candidate for press-swaging - - which would allow for variation in point design and weight - - if only dies could be constructed.
I'm sure that there are mechanical challenges to making hexagonal holes in a die, but I lack the engineering background to articulate just what those might be. Thought I'd throw it out there for your contemplation.