Just got a 257 Roberts to go shooting with. Plan on using it on varmints and up to whitetail and pronghorn size animals.
Could use some reloading data for a varmint size game and deer size the
loads would be under 300 yards. Open country hunting in desert lands.
I run 48gn of IMR-4350 with the Nosler BT and get close to 3200fps out of mine. Spark them with Fed210GM primers. Shot a 180lb Whitetail at about 300yds and it only took one step and then hit the ground. This load is just under max in warm weather in my rifle, so approach with caution.
By the way, which version of "improved" do you have, the RCBS or the A.I. ? The Rcbs has a 28deg. shoulder I believe while the A.I. version has a 40 deg shoulder. You will probably run into pressure signs faster with the RCBS version. Mine is the A.I. version. Hornady, Sierra, and Nosler all have some load info in their manuals. Hornady is the RCBS version and the others are the A.I. version. Good Luck.
I shoot a 25-06 and the 257 Bob IMP is close in velocity. As for deer bullets I really like the 117 Sierra. I very seldom have a deer move out of it's tracks when hit with it in the front shoulder. As for varmint bullets the 87 gr. Sierra is a bomb on groundhogs. These bullets are very accurate for me and they cost about half what the Nosler BT's do. As for powders everything I read says one of the 4350's or 4831 for you Bob IMP. There is just something special about most any .257 caliber rifle and DRIT killing of deer.