When you got one Weapon on your Back like a Sporter, and a second one in your Hands (like a Shotgun), and you want to jump over a Fence... Your Charactor stops running and does perform the "Shoulder Weapon" Animatoion. After that, your second Weapon is gone from your Hands and you can't see it for 10-30sec on the Ground. Sometimes its gone forever.
It happend to me twice in 12hours Playtime. I hope I can help you with that Information.
Keep up the good Work! Have a nice Weekend..