fig1 is a partially broken away side view of a smith & amp ; wesson model 645 . 45 caliber automatic pistol modified in accordance with the invention to fire a sub - caliber cartridge such as the nine millimeter or . 38 special cartridge . alternatively , an automatic pistol capable of firing cartridges of different calibers may be constructed in accordance with the principles of the invention . in a preferred embodiment of the invention , a pistol modified for firing sub - caliber cartridges utilizes all components of the larger caliber pistol , with the exception of the barrel and the recoil spring . in accordance with the invention , the pistol shown in fig1 comprises a frame 10 having a grip or handle 12 with a recess therein for receiving a magazine 14 to hold cartridges . mounted on the frame 10 is a barrel 28 and a breech block or slide 22 . the rear portion of the slide 22 serves to close the breech of the barrel , and the forward projecting portion of the slide surrounds the barrel 28 . the barrel 28 includes a chamber 16 for receiving a cartridge 20 . the barrel 28 also includes a downwardly depending lug 30 having an integral feed ramp 31 . a feed riser or feed spacer 18 , more fully discussed hereinafter , is affixed to integral feed ramp 31 . the firing mechanism of the pistol forms no part of the present invention and is not shown in fig1 with the exception of hammer 26 and trigger 24 . ln accordance with conventional smith & amp ; wesson automatic pistol construction , the barrel 28 is slidably mounted to the frame by means of an inclined plane formed by ribs machined into the frame 10 . upon firing , the barrel 28 and slide 22 recoil backward as a unit for a short distance . as recoil continues , the rear part of the barrel 28 is pulled downward and out of engagement with the slide 22 by ribs 11 on the frame 10 which mate with corresponding ribs 28a on the lower surface of the barrel 28 . these corresponding barrel ribs and frame ribs are not shown in fig1 . thus , after a short rearward movement of barrel 28 and slide 22 in unison , the rear part of the barrel 28 is caused to move downward , withdrawing the barrel 28 from engagement with the slide 22 , permitting the slide to continue to move rearward with respect to and separately from the barrel 28 . this separate rearward movement of the slide 22 effects the extraction and ejection of the empty shell in a manner known in the art . a recoil spring 23 causes the slide 22 to return in the forward direction , and during such forward movement a cartridge is withdrawn from the magazine 14 and pushed into the chamber 16 of the barrel 28 in position for firing near the end of its forward movement , slide 22 again engages the barrel 28 . in accordance with conventional practice , the barrel 28 and recoil spring 23 are configured so that the recoil spring 23 pushes against the upper rear portion of barrel 28 . barrel 28 is pushed against a slide stop pin , not shown , causing the barrel 28 to rotate around the axis of the slide stop pin . this rotation of the barrel 28 pushes the lower portion of the muzzle end of barrel 28 against the muzzle bushing 29 with an alignment force proportional to the force of the recoil spring 23 . this force , combined with the close fit of the slide 22 to the belled bearing surfaces of the muzzle end of barrel 28 , produces a consistent re - alignment force to locate the barrel 28 relative to the frame 10 . in a preferred embodiment of the invention , the recoil spring 23 is shortened to compensate for the lower recoil force generated by sub - caliber cartridges . alternatively , the recoil spring 23 is reformed to a lower strength using windings of different wire . fig2 a illustrates a conventional barrel utilized in a smith & amp ; wesson model 645 pistol . the barrel 27 , shown in engagement with slide 22 , provides a chamber 16 and terminates rearwardly in a downwardly descending lug 30 that includes 28a , and integral feed ramp 31 . the feed ramp 31 defines a first cartridge feed line suitable for feeding . 45 caliber cartridges . fig2 d and 2e show detail of the barrel 28 and barrel ribs 28a . fig2 b illustrates the configuration of the rear portion of a sub - caliber barrel constructed in accordance with the invention . in that embodiment of the invention , the sub - caliber barrel 28 is constructed using the monoblock portion of a conventional . 45 caliber smith & amp ; wesson automatic pistol barrel . the barrel 28 may also utilize a sub - caliber liner . the sub - caliber barrel 28 , which is surrounded by slide 22 , provides a chamber 16 adapted for holding a sub - caliber cartridge for firing . the barrel 28 terminates rearwardly in a downwardly depending lug 30 that includes an integral feed ramp 31 . the integral feed ramp 31 defines a first cartridge feed line suitable for feeding and chambering cartridges of the larger caliber , such as . 45 caliber . this first feed line is unsuitable for feeding sub - caliber cartridges , such as the nine millimeter cartridge . this is because the longitudinal centerline of a smaller diameter cartridge at the top of the magazine stack is higher than that of a larger caliber cartridge . consequently , the chambering force of the slide will tend to skew the smaller diameter cartridge downward instead of feeding the cartridge straight into the chamber . thus , a different , selectively higher cartridge feed line is provided by the inclined surface of feed spacer element 18 . in a preferred embodiment of the invention , in which the barrel assembly is constructed from a conventional barrel assembly , the feed spacer element 18 is welded or silver soldered to the integral feed ramp 31 of lug 30 . in another embodiment of the invention , the feed spacer element 18 is pinned to integral feed ramp 31 . either method may be utilized in conjunction with a conventional smith & amp ; wesson barrel construction . alternatively , as illustrated in fig2 c , a barrel assembly may be constructed specifically including element 18 , but without the original feed ramp 31 . such an assembly may , for example , be fabricated by conventional investment casting techniques or utilizing a monobloc construction . in all of these embodiments , the feed spacer element 18 selectively raises the cartridge feed line , so that sub - caliber cartridges feed into chamber 16 without skewing under chambering force . additionally , the elevated feed line prevents shorter sub - caliber rounds , such as the nine millimeter cartridge , from advancing toward the chamber prior to the appropriate point in the loading cycle . fig3 a is a horizontal section of the pistol of fig1 giving detail of the extractor and ejector utilized in accordance with the invention . a cartridge 20 in barrel 28 is shown in relation to slide 22 , breech face 34 , firing pin 32 and hammer 26 . in accordance with conventional practice , the rim of cartridge 20 is captured by the forward end of extractor 36 , which is mounted in adjacent extractor cut 40 , for removal from the chamber of barrel 28 upon rearward movement of the slide 22 . the spent cartridge is thrown clear of the pistol upon striking the ejector 38 . as fig3 b illustrates , the ejector 38 is preferably mounted pivotably to the frame , and travels in an upwardly angled impingement path 39 ( shown in dashed lines in fig3 b ) on the bottom of slide 22 this upwardly angled impingement path 39 provides positive contact between the pivoted ejector 38 and a sub - caliber cartridge 20 . in particular , the case of sub - caliber cartridge 20 is struck by the ejector 38 travelling upwardly along angled impingement path 39 . the slide 22 , which is substantially identical to the existing slide of the larger caliber pistol , preferably has a narrowed breech face 34 and an expanded ejector cut , as well as provision for the angled impingement path 39 described above . an ejector cut located adjacent to the feed rib , having a width equal to that of the ejector , and a length determined by the length of the cartridge being utilized , has been found to allow reliable ejection . the ejector cut may , for example , be 3 / 8 inches in length . in a preferred embodiment of the invention , the conventional ejector lug of the smith & amp ; wesson ejector is at least partly milled away to allow the ejector 38 to rise into the ejector cut milled into the slide 22 . the incorporation of this feature facilitates reliable ejection . in one embodiment of the invention , the conventional extractor pad is at least partially milled away , to provide more reliable extraction of smaller diameter rims . in an alternative embodiment , the standard . 45 caliber extractor is retained , and the extractor recess 40 is deepened . fig4 a - 4d depict a magazine utilized in a preferred practice of the invention , showing a sub - caliber magazine liner . in accordance with the invention , the magazine 14 , which is a conventional magazine utilized with cartridges of the larger caliber , includes outer magazine wall 44 , base 46 , feed lips 48 , spring 50 and follower 52 . the magazine 14 is fitted with a magazine insert or liner 42 adapted to hold cartridges of the desired sub - caliber , in a manner known in the art . another embodiment of the invention utilizes a conventional . 45 caliber magazine suitably modified to hold nine millimeter cartridges in particular , the feed lips of a conventional magazine are bent inward to slightly reduce feed lip gap , and the follower is altered by grinding or other means , to accommodate the shorter nine millimeter cartridges , as well as the rearmost extension of the barrel during the reloading cycle . a modified magazine of this type is capable of holding and feeding both . 45 caliber and sub - caliber cartridges , effectively providing a dual - caliber magazine . a magazine utilizing a sub - caliber insert or liner has been found to feed more reliably than the dual caliber magazine when loaded with cartridges other than full metal jacketed ( fmj ). the lined magazine illustrated in fig4 thus allows a wider latitude in cartridge selection and bullet shape , and in particular , permits the use of rimmed cartridges such as . 38 special . it will thus be seen that the invention efficiently attains the objects set forth above , among those made apparent from the preceding description . in particular , the invention provides a sub - caliber conversion unit for the smith & amp ; wesson model 645 which is inexpensive , requires a minimum of modifications or new parts for the larger caliber pistol , is simple to install and remove , and which maintains high accuracy levels . it will be understood that changes may be made in the above construction and in the foregoing sequences of operation without departing from the scope of the invention . it is accordingly intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative rather than in a limiting sense . it is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention as described herein , and all statements of the scope of the invention which , as a matter of language , might be said to fall therebetween .