I agree with the idea of fire forming with a 6.5 x 55 case and fast burning powder to get the the best fill out. I also agree that you shouldn't buy any headspace gauges at this point, due to the cost and the fact that they may not be what you need, and you may never use them again. Another option to see if you may have an excess headspace situation would be to cement round pieces of thin plastic shim stock of various thicknesses to your 6.5 x57 cases to see how much makes it difficult or impossible to close the bolt. It doesn't take very much to cause excess case stretching, or partial or complete head separations. At this point, it is impossible to know if you are dealing with an obscure standard chambering, a wildcat, or an over polished chamber. Here's a silly question... any chance of contacting the original owner to get some clues?
I'd still recommend slugging the bore too. And another chamber cast with very careful measurement at the correct time after casting is a good idea. You already have the materials.
