Something to be very mindful of when fooling with a long obsolete 95 year old cartridge that is going to amount to a modified, slightly undersize bullet, .25-06........

After forming and trimming of .30-06 & .25-06 cases with the .242 Vickers die and measuring I cannot help wondering if Manton and Kynoch did pretty much exactly that; necked down a .30-06 (.30 USG) case to nominally .242 specs? The case taper is a hair more and consequent shoulder diameter is a bit less, but base to shoulder is, as closely as I can measure, the same and shoulder angle is for all purposes the same as far as I can discern.
Yes, while the -06 case is very overbore for a ~.250" 100 grain bullet with the powders available in 1924; like all the cartridges meant for tropical use in "the Colonies" this large case at 2800'/sec meant comparatively lower pressures which were a desirable thing back then.

The adventure continues!
- Mike