ShootistPRS,
My first step in your shoes is to look at
the SAAMI drawings and measure the case diameters as well as the head-to-shoulder to see what is off. It could prove useful to obtain a box of commercial ammunition, fire a few of them to get before and after measurements and to compare the fired ones to what is coming out of your sizing die.
It is sounding like you may need a small base die for the first resizing of the fat cases, but I don't recall seeing one in .257 Roberts. Still, they make them for a lot more chamberings than they used to, so you might find some, and, obviously, one could be special ordered. However, before going to that investment, it is also important to note that tolerances on a lot of sizing dies aren't very tight. RCBS once sent me information on their small base dies, and I was much surprised to find 0.002" diameter tolerances. I assume that goes for their standard size sizing dies as well, which means something as simple as trying a different sizing die may fix things. Maybe you can borrow one.
Another thing you can try is to run a case into the sizing die, leave it in there for a count of five at the top of the stroke to let the spring relax a little, withdraw it and rotate it about 1/3 turn and repeating the resizing and pausing twice. I've taken -0.003" off the length of a .30-06 case doing that. I never measured what it did to the diameter, though.
The other thing is Mr. Guffey's trick of using an automotive feeler gauge slid between the case head and the bottom of its slot in the shell holder. Usually, there is room to raise it up to about 0.005" that way, so sizing will go that much further. Any way you make your cases go deeper into the sizing die, whether by feeler gauge or by reducing the shell holder deck or by grinding the die mouth back, the shoulders will come out set back more than you want. But after you fire them from that size once, you should be OK sizing them normally with the die set to produce your preferred shoulder setback specification thereafter. The trick is to get them loose-fitting once, then fire them. Tight cases don't normally cease being tight after firing, unfortunately, or you could just jam and shoot.
Though unusual, there are a few instances in which +P cases are made with thicker brass than standard cases are.
Starline's .45 Auto +P, for example. I didn't realize the .257 Roberts was another, but then I learn something new every day.
The cases that wouldn't fit your shell holder may have experienced excess pressure. Did you look to see if the primer pockets were loose?
The head of the .257 Roberts is the same as the .30-06 and other stepchildren of the .30-03, so you should be able to form brass from .30-06. RCBS forming die set #58118 makes 6.5mm-257 Roberts from either .257 Roberts or .30-06. You could first form 6.5 mm-257 Robers from .30-06 with that, then neck it down with bushings. You'll need to inside-ream or outside turn the necks to remove the extra neck brass thickness that will result.
Nathan said:
Redding makes shellholders meant to push the case in further...
It's the other way around. Their competition shell holders have taller decks in 0.002" increments to keep the case from going in as far as normal and pushing the shoulder back too far. This is to allow you to just set the shoulder back slightly but still make solid contact between the die and shell holder for consistency.
