I would love to provide some help, but you really need to know for sure the rifling twist. A very accurate barrel can be made that has a very slow twist to it, that barrel will simply be accurate with a lighter (more accurately SHORTER) bullet than the barrel with a faster twist. So, despite the barrel maker being good at his craft, he well could have used either twist rate depending upon whether the rifle was going to be used for Varmints and Targets or medium game like deer. A 1 in 14 twist barrel will typically shoot well with bullets of 87grs and under, although some will stabilize some 100 grain pills. A 1 in 10 will stabilize about any .257 bullet up to and including those weighing 117-120 grains.
For reloading data, just start with the starting loads for any 257 Roberts and add powder until your chronograph tells you that you have hit the book max speeds for a plain 257 Roberts, assuming that you do not experience any obvious danger/overload signs along the way such as flattened or cratered primers, stiff bolt lift etc. When one sees such signs, he is usually way past normal and safe pressures.
To check the rifleing twist, run a tight, slightly lubed patch into the barrel. Make a mark on the top dead center of the cleaning rod near the back of it. Push the patch through the barrel until the top dead center mark comes back to it TDC spot and measure how many inches the rod travelled before it came to a full circle. If it took only 9 or 10 inches of travel to make a complete circle, then you have a fast enough twist to use any bullet. If the distance between the TDC marks is 14 inches, you will be limited to 87 grain bullets with possibly some shorter 100gr bullets stabilizing well enough to shoot and hunt with.
Another method would be to load some lower velocity or starting loads for a regular 257 Roberts with 120 grain bullets then try to shoot groups at 25, then 50 yards. A 1 in 14 twist barrel will have those long bullets hitting the target somewhat sideways with an oval shaped hole, if they even hit the target at all.
I hope this helps some.
Last edited by safariman; 11/18/14.
