Either way you go all i can say is have some brass in hand before you build one. You can use 7x57, 6mm rem or 257rob brass. Good luck whatever you do.
I sure hope that all that read this post can listen to Dusty, it is sound advise. 257 Roberts, 6 Rem, and 7x57 brass is very hard to come by.
Also, rem made one run of the 257 Roberts in a long action, they are coveted. A long action on the 257 Roberts and the 6 Rem is a winning combo, and I have had a dozen or more 6 Rems, one 257 Roberts on a long action.
If you decide to go to a 6mm of sorts, the 95g partition is a game changer, I promise you that. While it does not have a High Tec B.C., the 95g partition has given me much more accuracy than the 100g partition. The 95g Partition is a bullet worth building a rifle around due to it's performance on small and Heavy bodied bucks. The 95g Partition has the ability to break heavy shoulder bones and keep traveling on quartering shots, breaking a rib bone upon exit. However, both 95g and 100g partitions give massive internal damage on 80-320 lb white tails that I have shot, along with great blood trails on those that traveled 30 yards, same with the 100g partition in the 25/06 caliber and 257 Weatherby. We hunted deer in Nebraska and Kansas where bucks run 200 lbs-320 lbs that we took. Shots were typically between 200-350 yards, and the 95g partition pole axed LARGE bucks, any angle you shot them from from a 6 Rem with a MV of 3150.
As Dusty has given a stern Uncle's type warning: brass on the 257 Roberts is very difficult to find...you dang sure better listen. If you want a "different" custom case and a 25 caliber, then consider the 250 AI off of Lapua 6x47L, this would be a no brainer, hands down home run.
It will leave a hollow place in your stomach if you have a custom barrel chambered in 257 Roberts, then have to settle in to the "SHOCK" of having little to NO good brass to work with. It would not be inconceivable that a guy have to beg/borrow/plead for used brass in a 257 Roberts. 6 MM Remington brass has not been made in 5-6 years at least.
Prvi Partizan brass is available in 7x57. However, when you neck down the 7x57, you are guaranteed a neck turning procedure in this process, so look before you leap....add $300 to the cost of your project if you are not set up to neck turn. All of these complexities shakes out to making a std 25/06, a very easy choice, and if you want a different case, then consider a 25/06 AI which feeds flawlessly out of a Rem 700. 100g partitions run 3500-3550+ in a long barreled 25/06 AI with R#22.
You see I have a preference for partitions, the reason is that I don't like running deer, and I do shoot for a center to high shoulder shot on broadsides. Large bucks chasing does take a lot of killing when they are pumped up, and a quartering shot facing and going away is not unusual at all. A Quartering shot taken on a buck that is walking or running away where the bullet has to enter the rear of the rib cage and break a shoulder as it exits is not that uncommon....better have a good killing bullet for that kind of shot. The vast majority of shots are taken under 350 yards, so analyze how you hunt, and pick a High BC bullet if your distances demand that kind of BC. Put a deer to leaking from two holes, with broke bones in between. Tipped triple shocks work well, but blood trails are "iffy". When hunting in jungle type situations, as in the South, a bullets ability to leave a Great Blood Trail is desirable. Caliber and impact velocity is critical for this issue with the triple shocks that I have shot, with light for caliber weight and impact velocity proving Key.
Sorry, I got to chasing rabbits on this, I had to learn the hard way, and I hope you can avoid some of the hard lessons that I had to learn.
