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 transversely to said first portion; 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, and said second portion of said track being configured to permit said pen needle assembly to be turned with said guide key being received in said second portion of said track with said proximal end of said needle being located proximally of said septum, characterized in that said track further having a third portion disposed transversely to said second portion of said track, and further comprising a spring disposed between said injector body and said hub, said spring configured to bias said hub distally away from said injector body, whereas said spring urges said guide key into said third portion of said track with said guide key being sufficiently moved through said second portion of said track. 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; 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 transversely to said first portion; 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, and said second portion of said track being configured to permit said pen needle assembly to be turned with said guide key being received in said second portion of said track with said proximal end of said needle being located proximally of said septum,