Firearm with interchangeable barrels and ammunition cylinders

In a firearm of the type having a frame for mounting a multiple chamber ammunition cylinder, said frame having a rear sight mounted thereon, and a threaded frame bore for removably mounting a barrel, a plurality of said ammunition cylinders with each of said cylinders being adapted for use with a different caliber ammunition, means for selectively mounting any one of said plurality of cylinders for revolution on said frame, a plurality of barrels, each said barrel having a different diameter barrel bore therethrough corresponding to a different caliber of ammunition associated with one of said plurality of cylinders, each barrel having a thread engageable in said frame bore and located at one end of said barrel for allowing selective mounting of each said barrel in said frame bore, longitudinal barrel extension means located on said one end of each respective barrel and being operative when said respective barrel is fully threadedly engaged in said frame bore for interfering with the mounting and revolution of any said cylinder in said frame which cylinder corresponds to a caliber of ammunition larger than the caliber of the barrel bore of said respective barrel, an improvement which comprises a plurality of interacting safety features including a barrel alignment mark located adjacent each said barrel thread on each of said barrels, a frame alignment mark located on said frame for alignment with said barrel alignment marks when each said barrel is fully threadedly engaged in said threaded frame bore to insure full threaded engagement and thereby the operativeness of each said extension means, a plurality of sight ribs, each said rib corresponding to a different said barrel, means for attaching each said sight rib to its corresponding barrel at a predetermined location on each said barrel to enable each said sight rib to be operatively oriented with respect to said rear sight for aiding aiming of said firearm when each said barrel is fully threadedly engaged in said frame bore, and means for inhibiting attachment of each said sight rib to its corresponding barrel when each said corresponding barrel is only partially engaged in said frame bore for allowing attachment of each said sight rib to its corresponding barrel only after each said barrel is fully threadely engaged in said frame bore to require that attention be directed to the degree of engagement between a said barrel and said frame bore in order to attach each said sight rib.