Or was it never meant for barrels to work as storage inside vehicles?

In T158518#2190158, @Geez wrote:

Hello Dispentry.
Player characters were never able to pick up open barrels unless there was a bug occurring that allowed for this to do so. Also the closed barrels can be placed into the M3S barrel slots without any issues.
Can you please describe what is the issue on your end in more detail?
Regards,
Geez