I have a Ruger Security Six and a Ruger GP-100 and have slugged both. Both OF MINE have .357" barrels, but the Security Six has 0.358" cylinder throats while the GP-100 has 0.357" cylinder throats.
I suspect that your Security Six will also have .358" throats. Seeing if a .358" lead bullet will pass through the throats in your cylinder will tell you if you can use them. If not, you could "save" the bullets by buying one of Lee's cheap bullet sizing dies and size them down to 0.357". That may actually be cheaper than buying a Lewis Lead Remover to get the lead out of your bore if you get leading from shooting undersized lead bullets.