When you have any launcher on the back, double click the "left ctrl" to enter the relaxing stand stance, then use the action menu to switch to the launcher, you will see the your avatar gonna play a step over action whilst leaving the rifle on the air.After it slowly completes the step over anim, then the rifle will be attract back.
Please check the photo.
http://cloud-2.steampowered.com/ugc/468668681963722269/49396061F70D36296C5F63CFDEAB1C885716BE31/