I noticed another player's sidearm was invisible whilst I was in Spectate mode, and he/she was shooting. When I click to spectate someone else and then come back, the sidearm could then be seen. See below:
PC Specs:
CPU: i7 4790k
GPU: GTX 970 4GB
RAM: 32GB DDR3 @ 1866mhz
HDD: Samsung 840 SSD