To playback a sample in one shot mode within KONTAKT 5 you must first enter the Edit Mode (spanner icon in the instrument section) and click on Wave Editor. You will see the sample waveform in the Loop Editor.

1. In the Sample Loop area set the Loop Mode to One Shot in the drop-down menu.
                                                                     
2. Then scroll to the bottom of the KONTAKT window and you will see a panel of Volume controls including ADHSR settings. Bypass the VOLUME envelope modul or, alternatively, set the Release parameter value accordingly.

After doing this the sample will play back correctly using One Shot mode.