The 257 Roberts is a 7X57mm case necked to 25 caliber. Same length. The 257 Roberts is already chambered in a long action, same as the 7X57mm, so there are no magazine problems. The only issue will be replacing the barrel. Douglas, ER Shaw, or McGowen will do rebarreling. Of the 3, only ER Shaw does rebluing, AFAIK. Or you can buy any barrel you want and have a local guy do it all.However, the 7mm Mauser OAL is 296 thou longer than the Roberts so you'll likely have a mag length issue.