wiki info "The rolling block is obtained from the mission "Empty Promises" which is given by Captain De Santa. You pick the weapon up out of a crate in a camp." I have yet to find the spawn location of the gun in Mp, but if i do (which i will) i'll re-edit this and post it's location.
wiki info "The Carcano Rifle can be purchased at the Gunsmith in Blackwater for $550."
You can also get the Carcano rifle from a Gang Hideout(multiplayer free roam)in NorthWestern Mexico, Nosalida Hideout.