referring to fig1 a preferred embodiment of the invention comprises an ampul 11 , an ampul receiver 12 , and a unit 13 housing the triggering and firing mechanisms . the ampul 11 ( shown here in somewhat fanciful form for illustrative purposes only ) consists of an elongated body having a relatively thin wall of glass , plastic , metal or other suitable material , having one or more orifices 15 at one end and an extended head 16 at its other end . a measured quantity of selected medicament 17 occupies the forward portion of the ampul 11 . a reciprocal piston 18 is positioned between the head 16 and the region containing medicament 17 . conventional o - rings or seals 19 form a gas - tight seal between the chamber 21 defined by the piston 18 and the medicament 17 . the head 16 contains an explosive primer or percussion cap 22 operatively connected to the deflagatory charge 23 whose composition , quantity , form and other characteristics are carefully chosen so that on detonation the gaseous products introduced into chamber 21 impart to piston 18 the precise impulse necessary to eject the medicament 17 through orifice 15 under sufficient pressure to insure its subcutaneous infusion . a closure 24 and retainer 25 of suitable materials are provided to protect primer 22 and charge 23 , respectively . as shown more clearly in fig5 a pair of arcuate recesses 26 are formed in the closure 24 in the underlying face of the head 16 . the purpose of these recesses 26 will be explained later . receiver 12 comprises an elongated barrel 29 having an annular shoulder 31 formed on its inner wall . a bushing 32 sized to fit snugly but rotate freely within barrel 29 is formed with an annular shoulder 33 which abuts shoulder 31 . one end of bushing 32 is provided with internal threads 34 of a relatively course pitch . the head 16 of ampul 11 is provided with external threads 35 of like pitch . the opposite end of bushing 32 is of reduced diameter and carries external threads 36 . the portion of bushing 32 between shoulder 33 and the external threads 36 presents a smooth cylindrical bearing surface 37 . an axial slot 38 is formed in the wall of bushing 32 . a sleeve 41 is secured to the inner wall of barrel 29 by brazing , soldering , welding or other conventional means . sleeve 41 is sized to permit bushing 32 to rotate freely . as best seen in fig4 and 7 , a portion of the wall of sleeve 41 is cut away to provide a generally helical caming surface 42 extending about a quarter of the sleeve &# 39 ; s circumference . the trigger and firing mechanism unit 13 is housed in a casing 43 slidingly mounted on a sleeve 45 which is internally threaded at one of its ends for secure attachment to the threads 37 on the end of bushing 32 . a slide pin 46 attached to the sleeve 45 and having its head extending into axial slot 47 permits casing 43 to move axially , but not to rotate , on sleeve 45 . an elongated firing pin 51 is mounted for axial movement within sleeve 45 . a powerful compression spring 52 acting between an annular shoulder 53 formed on the inner wall of sleeve 45 and an annular boss 54 formed on firing pin 51 urges the chip 55 of firing pin 51 in the direction of primer 22 . a pair of palls 57 at the ends of a pair of resilient , inwardly biased jaws 58 secured to the inner wall of sleeve 45 near its rear end fit into an annular groove 59 near the read end of firing pin 51 and restrain it against the thrust of spring 52 . a pin 61 having a frusto conical tip 62 is mounted axially at the read end of sleeve 45 with its elongated shank 63 extending slidably through a retainer washer 64 secured to the end of sleeve 45 . the end of shank 63 is securely fastened to the closed rear end of casing 43 . a compression spring 65 acting between the end of casing 43 and retainer washer 64 urges casing 43 in the direction away from receiver 12 . a cam pin 67 extends radially outwardly from firing pin 51 , passes through slot 38 in bushing 32 , and rides on the surface of the cam 42 formed in sleeve 41 . it will be observed that by grasping the receiver 12 in one hand and the casing 43 in the other and twisting the former through about 90 ° with respect to the latter , cam pin 67 may be made to following cam 42 , thereby forcing the firing pin 51 into the position shown in fig1 where pawls 57 restrain it against the force exerted by compressed spring 52 . preferably cam 42 is formed with a detent 68 at its end , which is adapted to retain cam pin 67 , thereby preventing the accidental or inadvertant release of firing pin 51 . as seen most clearly in fig6 an elongated safety pin 69 having an enlarged hed 71 extends through an axial bore 72 in bushing 32 . a compression spring 73 acting between the head 71 of pin 69 and the end of bushing 32 maintains the head 71 in contact with the annular boss 54 on firing pin 51 . fig2 and 3 illustrate the operation of the triggering and firing mechanism of the embodiment shown in fig1 . with the ampul 11 securely threaded into the end of bushing 32 , the firing mechanism is armed by rotating the receiver 12 and casing 43 as previously described until pawls 57 engage annular shoulder 53 and retain firing pin 51 in the position shown in fig1 . when the injection is to be given , the injector is armed by rotating receiver 12 and casing 43 in the opposite direction , thereby releasing cam pin 67 from detent 68 and returning the cam 42 to the position shown in fig2 and 3 . grasping casing 43 , the user places the tip of ampul 11 against the skin of the person to be injected . by applying sufficient force to overcome spring 65 , casing 43 is moved axially in the direction of the receiver 12 , thereby bringing the tip 62 of pin 61 into contact with the caming faces 75 of the pair of ears 76 formed on the inner faces of jaws 58 as pin 61 advances further , the caming action of tip 62 forces the resilient jaws 58 apart until pawls 57 are disengaged from annular groove 59 , thereby releasing firing pin 51 . under the influence of spring 52 the tip 55 of firing pin 51 is driven through closure 24 and into primer 22 with sufficient force to ignite the primer , thereby detonating charge 23 . the gases generated by the slow burning charge burst through retainer 25 , fill and build up pressure within chamber 21 , and drive piston 18 toward the tip of ampul 11 , discharging the medicament 17 through orifice 15 with sufficient velocity to penetrate the skin and infuse the subcutaneous layer . it will be noted that at the end of firing stroke , annular boss 54 on firing pin 51 has driven the end of safety pin 69 into the recess 26 aligned with bore 72 . with the pin 69 in this position , ampul 11 cannot be rotated for release from bushing 32 . to release ampul 11 , it is necessary to rotate the receiver 12 and casing 43 to return the firing pin 51 to the cocked position of fig1 . this in turn permits compression spring 73 to draw the pin 69 out of recess 26 , thereby freeing ampul 11 for rotation , disengagement from bushing 32 , and removal from receiver 12 . while not critical to the invention , this preferred feature serves as an automatic reminder to the user both to replace the spent ampul 11 and to arm the firing mechanism before each injection . referring now to fig8 , 10 and 11 , the embodiment shown here comprises an ampul 11 containing a medicament 17 , a gas operated piston 18 , a primer 22 and deflagrant 23 , a receiver 12 including a barrel 29 mounted for axial and rotary motion to a unit 13 housing triggering and firing mechanisms . this embodiment differs from the previous one principally in the means employed to secure the ampul 11 within the receiver 12 . in this embodiment , instead of the threaded , radially symmetrical head 16 previously described , the ampul 11 here has a head 81 of roughly rectangular cross - section . likewise the inner walls 82 of the barrel 29 are of the same generally rectangular cross - section , being of greater extent along a first radial axis then along a second at right angles to it . the inner walls of the rear portion of this receiver 12 define a generally cylindrical chamber dimensioned to receive the head 81 of the ampul 11 but , by virtue of the &# 34 ; flatened &# 34 ; configuration of the mouth and throat of the receiver 12 , having a pair of shoulders 85 extending radially outwardly of the inner walls 82 in the direction of the shorter cross - sectional axis of the mouth and throat of the receiver 12 . to insert this ampul 11 , its head 81 is rotated until its major axis is aligned with the major axis of the mouth and throat of the receiver 82 . when fully inserted into receiver 12 , the ampul 11 is then rotated 90 ° to bring the shoulders 86 extending outwardly of the ampul walls 14 in the direction of the greater axis of head 81 into contact with the shoulders 85 of the chamber at the rear of receiver 12 . this &# 34 ; key &# 34 ; arrangement serves to lock the ampul 11 tightly in place in receiver 12 . in this embodiment a recess 88 is formed in the rear of the head 81 and a spacer 89 is positioned within it . when the injector in this form has been fired , the spent ampul 11 can be removed , but as long as the firing pin 51 remains in its uncocked position , the spacer 89 comes in contact with the tip 55 of the firing pin 51 and prevents a new ampul 11 from being fully inserted into the receiver 12 . this arrangement serves the same purpose as the safety pin 69 of the previous embodiment , that is , to remind the user to arm the trigger and firing mechanism to make the injector ready for use . preferably , a pair of stop pins 91 are positioned extending axially rearwardly into the chamber at the rear of the receiver 12 to insure alignment of shoulders 86 with shoulders 85 . naturally , in this embodiment of the invention there is no need for the safety pin 69 , compression spring 73 or bore 72 . in all other respects the two embodiments are substantially identical .