Q: I intend reloading for a 7.7mm Jap rifle for use in military rifle competition. One reference lists the bore diameter as .303″; groove diameter .315″; groove depth .006″. I question these figures. Can you tell me what the proper bore and groove dimensions as well as groove depth is for the Japanese Model 99 Arisaka rifle in 7.7x58mm rifle? What diameter bullet gives the best accuracy in these rifles?
A: It is possible that some Jap rifle barrels may have those dimensions, but I have found that groove diameter is more likely to be near .312″. Reloading Manuals recommend use of .311″ to .312″ diameter bullets. A friend of mine who is a keen gunsmith- experimenter tells me he has found that .308″ bullets will give the finest accuracy in the 7.7mm Jap rifle. Use of a larger diameter bullet may well result in unnecessarily high pressures.