What have I got here?

I posted earlier looking for larger bullets for my Bolo C96 which, by the book, should use .309 bullets, but may like slightly larger bullets, i.e. .311 or so. But, in mine, those bullets fall straight through the barrel. So, I tried larger .32 caliber bullets. They fell straight through also.
I do have a couple C96s that were converted to 9mm. So, I tried a 9mm and it easily fell half way down the barrel! A dummy 9mm chambered. At the muzzle, it measures .346. A 9mm I had handy measured .342.
I bought the gun several years ago as a 7.63X25 and fired it as such just afterwards. I don't remember how accurate it shot, but if it was bad I probably attributed it to being my fault.

What is my next step? Should I consider it a 9mm conversion?

The magazine will feed both 7.63 and 9mm without modification. There a trizillion fake "red nines" out there.
I took one of the 9mm C96s out and did the same test with a 9mm bullet. The bullet dropped almost exactly as far into the barrel as it does with the gun in question, and the muzzle measures within .002 of the questioned gun.