If you handload, the 7.7X58 is easy to form brass from 30-06. Some 7.7 chambers were somewhat large and would swell the case body when fired, check the dimensions of a fired 7.7 case against that of a fired 30-06 case and see if your 7.7 chamber is larger. My 7.7 is tight and 30-06 cases work well with long life and good performance. To form cases, I lube and size 30-06 in Lee 7.7 die and trim back using Lee case trimmer setup and proceed as I would with any other bottleneck rifle case.