I'd follow Lancaster's advice, slug the bore and do a chamber cast (or have it done). That is the surest (and simple) way to get an accurate model of precisely what your bore and throat dimensions are.

All 'proper' M1900 Mannlicher Schönauer rifles and carbines were chambered for the 6.5X54MS. Is your scope mount dovetailed through the front ring, or is the model designation still visible?

The original chambering for the M1908 model would be the 8X56MS, as such,:

If you have a modified M1900 it's anybody's guess what feeds it, so straight back to Lancaster's advice.

Perhaps this may be helpful?: