Haven't tried it again, but i expect it to be reproduced following these steps:
1.: Reach the town of Turovo and find the lake.
2.: In the north-east of the lake there is a gap in the small stone wall, from where you can reach the water.
3.: Step into the lake until you start swimming, with your backpack on your back and your weapon aswell.
4.: Swim a little while and observe your backpack and your gun disappearing into the lake.
5.: If you have some sidearms (like pistols/sawed off shotgun/etc) in your backpack these will also be lying on the ground of the lake.
6.: Good Luck to recover your stuff.