Unless that rifle has been rebarreled, there's pretty much a guarantee it has the 12 twist. All the German made rifles had it. It was my first thought when I saw your target, as your bullet holes are slightly keyholed, telling me the bullet isn't stabilized by your twist, and is tumbling. That you got two on paper out of four is pretty impressive in and of itself.
For the 12-twist, if you're just dying to shoot monolithic bullets, I'd recommend stepping down to 100-110gr monoliths (not sure what's available from Barnes/Nosler/Hornady for 257cal, but somebody made a 100gr at one time). Alternately, since you know the accubonds work well, you could stick with them. For anything you should be hunting with the 257 (antelope, deer, coyotes, hogs, etc.) those accubonds will do the trick nicely, from close to far out there. When you go after elk, take the 300. Sure, a 257 will kill elk, if the shot placement is perfect. But for a hunt you may not do every year, take a rifle which will give you the margin for quartering shots where you're going to plow through a lot of bone, so you don't lose an opportunity to having too little gun. There is no such thing as too much gun, except for too much gun for a person to shoot accurately.