I have consistently encountered a similar problem with barrels in the Containers collection of the M3S. Items in the Inventory collection can be removed by accessing the side toward the rear corner of the truck. I have yet to find a location to be able to remove any barrels once added. I can't even add items to the barrels once in the truck either.

If anyone has had any success in removing barrels and/or filling them in the M3S, please tells us your secret...

Go to rear of truck drag barrel down to normal inventory of truck, walk to side and can then remove from side into hands or on ground. That is only solution I've figured out "if barrel is empty", if barrel has items in it I haven't had any luck or know what to do and barrels get permanently stuck in truck for me, but my friend has been able to get barrels out of truck with loot in them a few times by moving truck.

In T156838#2164986, @Drift-13b-88 wrote:

Go to rear of truck drag barrel down to normal inventory of truck, walk to side and can then remove from side into hands or on ground. That is only solution I've figured out "if barrel is empty", if barrel has items in it I haven't had any luck or know what to do and barrels get permanently stuck in truck for me, but my friend has been able to get barrels out of truck with loot in them a few times by moving truck.

Thanks for the advice! I will give this a try. I have had some luck on occasion with removing items out of the barrels while still on the truck. The biggest issue was not being able to do it reliably. Off to do some more testing...

It worked! I was able to remove all empty barrels from the truck. Thanks!