25x284 vs 257 Roberts barrel???

[Dalton Masterson]

I have a mauser with a replacement barrel. Reciever is marked 25x284. Sizing dies that came with gun are the same. Loaded ammo specs out correctly. When fired, primers back out, and neck moves forward about .042". Thought it was headspace problems, so took off barrel. It is marked 257 Roberts on chamber end!! Question is, without me having a 257 Robert round laying in front of me, is it possibly a 257? Or was the barrel rechambered into 25x284? Headspace seems about perfect on fired cased, but pretty short on resized 25x284. Thanks, and sorry for the long post. DM

[gunnut69]

Sounds like a headspace problem... Markings not withstanding a 25/284 would not even chamber in a Roberts chamber.  Check the headspace and you'll likely find it long..  The stamps on the end of a barrel are usually faced off when it's chambered..but it's not mandatory.. measure the headspace and lathe back the shoulder and extension a thread length and recut the chamber. Should produce about what the 25-06 is. In a shorter casing!