If you put these 2 commands in the barrel code:
- NPCSolidProps (Solid, Pushable)
- NPCMoveProps (ArndWall, ThruPC)
The barrel will be pushable on the belt but slowly enough so that it can't be pushed further than the belt limit at the bottom.
Immediate edit: ...I thought it was a solution, but not really. If you're quick enough you can push it lateraly from left to right so that you can bypass it... And if you push it without running it walks through you... Well, useless post sorry^^