hallobay's .257 Model 70

It sounds to me like the rifle has been rechambered to Ackley Improved and the chamber cut too deep. I think a chamber cast is in order. At least that's what I'd do before I'd start shipping it around in hopes of finding a gunsmith that was knowledgeable on those old M70's.

Another thing I might try: Take an empty case, neck it up to .27 or .28 caliber, neck it back down incrementally until the bolt closed on the case with some resistance. Load that case conventionally with a starting .257 load and fire it. That case would show me what I had. If it is, in fact, a too deep .257 AI, the foregoing method will need to be used to make cases for it or the barrel will need to be set back and rechambered. I have a couple of .30/06's with generous headspace and use the above method to make cases for them. My old M70 in .300 H&H develops incipient case separations so I use that method for it, also.