I would start by pushing a soft lead slug down the barrel, then mike it to see what the bore diameter is. If it's .311, then there's a fair chance it is 7.65 Argentine. But there's no guarantee it hasn't been rechambered for a different cartridge, .30-06 for example. Only a chamber casting will tell for sure.
The misfire could be due to excessive headspace, a common problem with surplus rifles. One really shouldn't shoot a surplus rifle without first checking headspace.