Jim,
YES, it is safe to load and shoot those Hornady 250 grain slugs.
Make sure that your case expander plug gives the tight bullet-to-case fit needed for this cartridge. A solid roll crimp is important too, especially the slugs are a bit loose for your cylinder throats.
Most revolvers will tolerate slightly undersize bullets because the bullet tends to "slug up" after leaving the cylinder and enters the forcing cone in the barrel. For that jacketed bullet, the barrel groove diameter (nominally 0.452") is still fine for good accuracy.
