This happens to me ALWAYS when I log in to the current experimental branch 0.42.116002. (Win 7 Home Premium)

I keep the two guns in the inventory until I use anything with my hands, so for instance when I eat / drink something or when I use a melee weapon. Then the second gun "falls down" and appears on the ground / floor.

I have tested and reproduced this bug with:
*shotgun
*new .22 hunting rifle

EDIT:
Also with:
*SKS
*mosin
*m4
*blaze

> works with every gun for me

Vote up!