No, it can't be done and yes it can but not with headspace. The SP-101 was originally designed for 38 specials and the frame window isn't large enough for the 357 with anything other than the 125 grain slugs. This is why you'll find 357 guns marked 125 grain ammunition only. After a short period of this nonsense, Ruger opened up the frame window so a cylinder of 357, 180 grain loads would function correctly in it. When this happened, the 38 special was dropped and the SP-101 became a 357 mag gun that would shoot 38 specials. In a nutshell, you can open up the chambers to shoot 357 mags but with only 125 grain bullets or wadcutters. A factory 158 grain load wouldn't allow the cylinder to spin from sticking out the end and cutting a 38 special gun would ruin it's value to anyone who wanted to collect it. I have a fixed sight 38 special with laser grips and absolutely love it. I load it to plus-p levels and that's so close to magnum loads, it's not worth measuring the difference. Don't ruin a good thing, buy a 357 SP. Yeh, headspace isn't the term you want to use since these do headspace on the rim, both of them.