Jack,
The .38spec. wadcutter is the most accurate of all the .38spec. ammunition. It has the longest bearing length touching the lands and grooves of the barrel. It is normally loaded relatively mild to also reduce recoil. The standard TARGET load is 2.7gr of Bullseye powder with a 148gr wadcutter. Wadcutters CAN be loaded up hotter than that and is recommended by some folks as an excellent self defence load for snub nose revolvers. If you use a lot of .38spec. wadcutter ammo in your new .357mag., you must clean the chambers often because the .38spec. is shorter than the .357 and will leave a ring of "crud" in the chamber that makes the subsequent use of .357 difficult to fit in the chamber. Some folks load wadcutters in .357 cases to combat this but I do not believe it is loaded this way commercially. Good Luck with your new .357 and let us know how you are doing. Quantrill