A medical injector comprising: a pen needle assembly including: a hub; a needle fixed to said hub, said needle having a distal end, formed for insertion into a patient, and a proximal end; an injector body being tubular in shape with a distal end and a proximal end; and a reservoir for medicament sealed by a septum, said reservoir being encased in said injector body; wherein, a guide key is fixed to one of said hub or said injector body, a track being defined on the other of said hub or said injector body, said track having a first portion extending parallel to a longitudinal axis of said injector body and a second portion disposed at an angle to said first portion, and a third portion disposed at an angle relative to said second portion of said track, and parallel to said first portion of said track; and, wherein, said hub is formed to be telescopingly mounted onto said distal end of said injector body with said guide key being accommodated in said track, and wherein, with mounting said hub of said pen needle assembly onto said injector body, said first portion of said track being configured to receive said guide key and to guide said guide key in a straight line across sufficient distance to permit said proximal end of said needle to fully pierce through said septum without rotation of said needle, said proximal end of said needle being guided to a location proximal of said septum, characterized in that said second portion of said track is angled with respect to said first portion of said track, such that said hub rotates as said hub is moved into engagement with said injector body by movement of said guide key into said second portion of said track. a hub; a needle fixed to said hub, said needle having a distal end, formed for insertion into a patient, and a proximal end; an injector body being tubular in shape with a distal end and a proximal end; and a reservoir for medicament sealed by a septum, said reservoir being encased in said injector body; wherein, a guide key is fixed to one of said hub or said injector body, a track being defined on the other of said hub or said injector body, said track having a first portion extending parallel to a longitudinal axis of said injector body and a second portion disposed at an angle to said first portion, and a third portion disposed at an angle relative to said second portion of said track, and parallel to said first portion of said track; and, wherein, said hub is formed to be telescopingly mounted onto said distal end of said injector body with said guide key being accommodated in said track, and wherein, with mounting said hub of said pen needle assembly onto said injector body, said first portion of said track being configured to receive said guide key and to guide said guide key in a straight line across sufficient distance to permit said proximal end of said needle to fully pierce through said septum without rotation of said needle, said proximal end of said needle being guided to a location proximal of said septum, characterized in that said second portion of said track is angled with respect to said first portion of said track, such that said hub rotates as said hub is moved into engagement with said injector body by movement of said guide key into said second portion of said track.