The bullet may be .30 cal ball that was turned on a lathe to expose the core. This was commonly done to create makeshift hunting ammo. When the kinetic bullet puller was used the core was loosened.
This thread does make one wonder though why a 7.62x57 Mauser or 7.62x58 Jap based wildcat never originated. The 57 or 58 mm case would be a perfect taper and fit for the full range of Mauser actions, from 1891 Argy's to 1898's to M48 Yugo's. Pressure concerns wouldn't be an issue due to the need for handloading and if commercialized could be loaded safe for all action types.
The argument has always been about the availability of .30-06/.308 Win commercial/surplus ammo and that is reasonable. But it pretty much limits one to using a standard length M98 action and frequently having to make feed rail, ramp and mag length mods for the '06. The .308 frequently has feeding problems(due to a different taper and short case) and is safety limited to a M98 design. If we used the same argument, the 7mm/.308 and the .260 Rem would have never seen the light of day. The .257 Roberts and the 6mm Rem were wildcat derivatives of the 57 mm Mauser parent case. With the abundance of Mauser actions, brass, .30 cal barrels and bullets, it is curious to me why this "wildcat" never got a start.
