I realize real men don't read the instruction manuals but your question is clearly covered in the Ruger Instruction Manual;
RUGER® SP101® revolvers are designed for use with cartridges of the
correct caliber which are manufactured in accordance with the U.S. Industry
Standards. Be certain to use proper cartridges of the correct caliber. See page 21
(“Care and Cleaning”) for important information on chamber cleaning.
Please refer to the Notes below regarding proper ammunition for the available
SP101 calibers.
Notes on Caliber .38 Special: .38 Special caliber revolvers are designed
specifically for modern higher-powered factory loaded cartridges, including .38
Special and .38 Special + P ammunition.
Notes on .357 Magnum caliber: This revolver is designed to fire factory loaded
.357 Magnum, .38 Special and .38 Special + P ammunition.
Special Ammunition Notice for .357 Revolvers marked “125 GR. BULLET”
only: Some revolvers have slightly shorter cylinders and frames and are
therefore limited to the use of .357 Magnum ammunition with a maximum
bullet weight of 125 grains and an overall loaded cartridge length of not
more than 1.57 inches. These revolvers are roll-marked with “125
GR.BULLET” on the right side of the barrel.
Ammunition containing heavier bullets or of a longer length may not permit
the cylinder to close or rotate! You may not even be able to fire the revolver if
you try to use longer or heavier bullets. Be sure the ammunition you wish to
use in the revolver will chamber freely, and will permit the cylinder to open,
close and rotate readily. Do not assume any ammunition will work without
checking it first.
All other .357 SP101 revolvers can use all standard factory .357 loads.
http://www.ruger.com/products/_manuals/sp101.pdf