Choke tube assembly
BACKGROUND OF THE INVENTION 
     This invention relates to firearms and deals more particularly with an improved choke tube assembly for a shotgun. 
     Heretofore, shotguns have been available with interchangeable choke tubes which enable a shooter to select the most appropriate barrel/choke tube combination which will provide an appropriate shot pattern suited to the shooters particular requirements. However, the increasing use of ecologically friendly steel shot in shotgun ammunition has imposed somewhat more stringent requirements for the manufacture of choke tubes. The use of more costly materials which are somewhat more difficult to machine has resulted in a substantial cost increase in the production of choke tubes. 
     Accordingly, it is the general aim of the present invention to provide an improved choke tube assembly which retains all of the advantages of existing choke tubes and may be manufactured at reduced cost. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention a gun choke comprises a choke assembly which has a tubular rear part and a separate tubular front part selected from a plurality of front parts. The rear part has a substantially uniform outside diameter throughout a substantial portion of its axial length and a coaxial forwardly converging conically tapered rear bore portion which defines a portion of a forcing cone. Each of said front parts has a substantially uniform outside diameter throughout a substantial portion of its axial length, an outside diameter greater than the outside diameter of the rear part and a forwardly diverging conically tapered front bore portion which further defines the forcing cone. Each of said front parts also has a generally cylindrical choke bore which is contiguous with the forward end of the conically tapered front bore portion and opens through the forward end of said front part. The choke bore of each front parts is of a different diameter. A means is provided for releasably retaining the choke assembly in assembled relation to and within a choke receiving bore formed within and opening outwardly through the muzzle end of an associated barrel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an axial sectional view through a barrel assembly embodying the present invention. 
     FIG. 2 is an axial sectional view through the rear part of the choke tube assembly shown in FIG. 1. 
     FIG. 3 is an axial sectional view through another front part embodying the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     In the drawing and in the description that follows the invention is illustrated and described with reference to an improved shotgun barrel assembly designated generally by the reference numeral 10. The illustrated barrel assembly 10 includes a shotgun barrel 12 and a choke tube assembly, indicated generally at 14, embodying the present invention and releasably secured within the forward or muzzle end of the barrel 12. 
     The illustrated shotgun barrel 12 has an axially elongate gun bore 16 extending through it and terminating at a choke receiving bore, indicated generally at 18, which opens through the forward or muzzle end of the barrel. The choke receiving bore 18 is further characterized as a generally cylindrical stepped bore and has a first bore portion 20 communicating with the forward end of the gun bore 16 and a second bore portion 22 forward of and communicating with the first bore portion. At least the second bore portion defines an internal thread, but preferably, and as shown, both the first and second portions of the choke receiving bore are internally threaded, the pitch diameter of the thread on the first bore portion 20 being somewhat different than the pitch diameter of the thread on the second bore portion 22. The illustrated gun barrel 12 also has a smooth generally cylindrical third bore portion 24 of relatively short axial extent which opens through its forward or muzzle end. 
     Considering now the choke assembly in further detail, the illustrated choke assembly 14 is formed by two separate tubular parts and includes an inner or rear part 26 indicated generally at 26 and an outer or front part selected from a plurality of front parts and designated generally by the numeral 28 in FIG. 1 
     The rear part 26 has a generally uniform outside diameter along a substantial portion of its axial length and a coaxial conically tapered and forwardly converging rear bore portion 30 which defines part of a forcing cone, hereinafter further discussed. 
     Each front part has a generally uniform outside diameter along a substantial portion of its length and is somewhat greater in diameter than the rear part 26. Each front part further includes a coaxial conically tapered and forwardly converging front bore portion 32 which further defines the forcing cone indicated by the numeral 33 in FIG. 1. The front bore portion 32 may have the same taper per unit length as the rear bore portion 30. However, in the illustrated barrel assembly 10 the taper per unit length of the front bore portion 32 is somewhat greater than that of the rear bore portion 30. Each front part also has a generally cylindrical choke bore 34 which communicates with the forward end of the front bore portion 32 and opens through the muzzle end of the front part 28. The major diameter of the front bore portion 32 is somewhat greater than the minor diameter of the rear bore portion 30. The major diameter of the rear bore portion 30 is, in turn, somewhat larger than the diameter of the gun bore 16. Diametrically opposed key slots 36, 36 (one shown) are formed in the wall of the front part 28 and open through the forward end of the front part for receiving a key used to assembled the front part with and remove it from the barrel 12. 
     At least the front part 28 is externally threaded along a portion of its axial length to engage threads defined by the second front bore portion 22 to releasably retain the choke tube assembly 14 within the choke receiving bore 18. Preferably, and as shown, both the rear part 26 and the front part 28 are externally threaded to engage threads within the respectively associated first and second portions of the choke receiving bore 18. The pitch diameter of the threads on the front part differs from the pitch diameter of the threads on the rear part so that the choke parts cannot be improperly assembled with an associated barrel. 
     The front part 28, shown in FIG. 1, is typical of the front parts which may be used in combination with the rear part 26 to form a choke tube assembly. However, another front part which may be used with the rear part 26 is shown in FIG. 3 and indicated generally at 28a. Portions of the front part 28a which correspond to portions of the front part 28, previously described, bear the same reference numeral and a letter &#34;a&#34; suffix. 
     The illustrated front part 28a is identical in most respects to the front part 28, but has a somewhat smaller choke bore 34a than that of the previously described front part 28. The taper per unit length of the front bore portion 32a is also somewhat greater than that of the previously described embodiment. 
     It will now be apparent that numerous barrel/choke tube combinations may be attained to provide a full range of shot patterns by simply substituting one front part for another.