You'll probably find that the bore is .309. I have a Rossi single shot that has a .309 bore and it's chambered in 7.62x39. I believe gun manufacturers keep their cost down by making the bores common between 30 and 32 calibers. A few thousands isn't going to blow the gun up but the accuracy will be in the toilet. I assumed that the bore on mine was at least .311 so I was sizing at .312 but the thing was shooting all over the place at 25 yards. Now, I size at .309 with 14 grains of 4756 and I can squirrel hunt with it.
Also, if you're going to use cast bullets, the bore must be clean of all copper fouling. Powder fouling isn't so much an issue but the copper has to come out otherwise, you'll have bad leading.