That sounds like you're referring to the lower receiver rails/edges - as most easily viewed by looking downward through the ejection port, toward the inside of the left receiver wall, with the bolt back.
If the rail/edge had cracked, and you were able to then snap it off - doesn't ring true with me, of a normal condition, even with heavy use/abuse.
I would only think it safe to fire, after the rest of the action had been professionally checked for further cracks (magnafluxed) and passed a (Brinell) hardness test - in case the heat-treat has been effected by a fire or other heat source in the gun's past.