I want to turn this barrel into a moving platform that rotates as it moves
How do you want it to move? you want it to run along a spline and then loop back around to the beginning? for the rotation part simply add the function to event tick from a blueprint of add local rotation and plug in your item you want to rotate. Adjust to your liking from there on
As for assuming you want the object to run along a spline then just attach it in a spline in a bp and run a timeline for X seconds where X is your time for how long you want the barrel to go around the spline, next you run that into the Alpha part of a lerp where the A part remains a 0.0 and then plug in the get spline length from the spline into the B part. Remember that this must remain on world space so make the Locations of the BP itself go to (0.0,0.0,0.0) with the world and the blue print you can just use a target point to plug in data so you can work faster, lastly you want to get (location at distance along spline)(You get this from the spline itself) plug in the return value of the lerp and keep the coordinate space to world, and then plug that into set world location of the object that you want to move around such as your barrel. Plug in the update to the set world location and then run a delay of 0.1 seconds into the finished section of timeline and right back into the play from start.
Assuming you want this object to continue to rotate forever in the same path of X location for X seconds.