When it comes to casting body parts, you don't ever want to use one of those industrial silicones found at places like Home Depot. Why not? Because they're NOT SAFE for your body. Aside from that, they won’t be able to capture any fine details, they're relatively unstable, and they'll shrink and/or deform during cure.
They're not body safe
The tubes of silicone you see in hardware stores are indeed silicone, but they're what's called condensation-cure or tin-cure silicones, not addition cure silicones like those used in body casting. Though based on the same core structure: silicone+hydrogen+oxygen+carbon, the way each cures or hardens makes them essentially two entirely different materials.