Use the .312 it's better than .310 and you're starting low and working up anyway, so you're not in any danger there, plenty of .308 bore 7.62x39 tubes have had a 123 shoved down their throats with no ill affect, besides which, .001 larger than bore gets a good grip on whatever rifling is left.Just to verify...
I slugged my bore at 0.311". Using a 0.312" jacketed bullet is probably not advisable, correct? I know lead is fine in most cases, but the copper jackets may be too tight?