Sambarman,
As Nakihunter stated, if your bore slugs out at .266 or more, you may wish to try Hornady Carcano .268 bullets, assuming availability. You should get better accuracy than with .264 bullets, and better velocity with mild loads for an old gun.

Geoff

I haven't slugged my old barrel yet and if finding one of the RWS bullets came out at .268 it would only suggest going to bullets of that diameter may not be such a good idea. I say this because none of of my long 159gr RWS bullets measures more than .261 beyond the case neck.

The MS rifling is the strangest of seven rifles I can look down, from .22rf to .450/.400. In all other cases the lands appear to be about 1mm wide, the groove between them perhaps 3-4mm; in the MS the lands are more like 2mm wide and the groove not too much wider than that. This suggests to me that there may indeed be a need for the projectile to squish out into the narrow but canyon-deep grooves.

The grooves are quite dark after 87 years but if there is any young gas getting past the bullet, it doesn't seem to hurt accuracy, which I suspect is close to two inches even with open sights.