zim,
i also ponder such things, and the x64 is a bit similar.
i surmize that these rounds might have been originally designed to fit in 8x57 magazines, where many modern mags are 30/06 length.
coal is critical in 2 areas, 1) afore mentione mag length, and 2) ogive touching lands.
for hunting, as long as rounds fit the mag and clear the rifling slightly that is all that matters.
some factory rifles are throated seperately to chambering, and in some cases this process is overdone, probably on monday mornings and friday afternoons.
ruger used to be known for this particularly.
the long bearing surface of the 300 gn swift aframe 9.3mm bullet needs to be taken into account with coal and rifling contact.
the hornady bullet in your load might behave differently terminally than at factory muzzle velocity.
they could be a better killer of big game than the 286 partition, and would certainly be more predictable in terminal performance.
i bought a 9.3x64 throating reamer for my gun to extand the throat a little in order to use the 300 gn swift at full mag length with about 0.030" jump.the throat angle in about 1/2 degree, an angle that is condusive to not having high pressures.
i am thinking of trying a 284 win barrel at this angle in an fclass rifle one day.
bruce.