I have a SMLE sporter, evidently made in India, with the marking ".315" on the breech along with the proof marks. I assumed that the bore had been opened to .315" to comply with the law prohibiting military calibers and simply loaded .303 British brass with .315" bullets I sized down from .323". It seems to work fine, at least to the extent that the bullets do not keyhole. The sighting system is so primitive that I can't really comment on the accuracy.

I wonder if your rifle might have the same chamber as mine.