Leave the oval bore for a minute: the 297/25 calibre is common in Rook Rifles. The groove diameter with conventional rifled bores is .25. problems arise with modern .25 moulds, which are all .257 diameter as cast. For the rifle to shoot well, they must be sized down to bore diameter.
Back to the oval bore: this was used almost exclusively by Charles Lancaster. The bore should be slugged and the bullet sized to .001" under the maximum diameter for best accuracy. I use a Lyman .25acp mould.The bullet is lubricated before and after sizing, and with 1.8 grains of bullseye or .5cc of Swiss no 2 black, both loads shoot to the same point at 50 m. Cases are easily formed from .22 Hornet brass.
Hope this helps,  
regards,
Fred.