When the SP101 was first introduced it was chambered in .38 Spl. IIRC the next year they came out with the .357 Mag version. But the first production runs of the .357 Mag SP101 had the same shorter cylinder as the .38 Spl. version and the side of the barrel was inscribed with the disclaimer "125gr .357 Mag Ammo Only" (or words to that effect). They wouldn't reliably chamber standard 158gr ammo. The next year Ruger lengthened the cylinder, now the SP101 will chamber any SAMMI spec ammunition.... but with little room to spare.
With that said, it is easily possible to load ammo that's longer than SAMMI specs. I suspect that's what Pro-Load has done with the ammo you bought. It's an old handloader trick: by loading the bullet long more room is created in the case for powder - allowing for higher velocities at standard pressures. I use to do it myself when loading hunting ammo for my S&W 686 (most full sized revolvers have cylinders 1/16" or so longer than SAMMI requires). Hornady and others even make .357" bullets with an extra crimping groove to facilitate loading ammo with extra long OAL. -- Kernel
