A pen needle assembly is provided herein for use with a medical injector containing an injector body and a reservoir sealed by a septum encased in the injector body. The pen needle assembly includes a hub and a needle fixed to the hub. The hub is formed to be telescopingly mounted onto the distal end of the injector body using a track that accommodates a guide key. When mounting the pen needle assembly onto the injector body, a first portion of the track is configured to receive the guide key and to guide the guide key in a straight line across sufficient distance to permit the proximal end of the needle to fully pierce through the septum, without rotation of the needle. A second portion of the track is configured to permit the pen needle assembly to be turned with the proximal end of the needle being located proximally of the septum.
