The arctan is usually “atan” in programming languages, and this holds for Blueprints as well. Atand will give you a result in degrees, so you can go for this one.
I’m honestly not sure how you can get the gravity; it seems there is a node to get the world gravity, but it requires a world settings reference, and it appears to be different from the gravity set in project settings.
For the time being, you can just use 981 (or whatever value you have). Also, be careful with units: the calculation is done in meters, but UE4 works in centimeters. I don’t think it has an effect here, but better check anyway.
As for the ProjectileMovement, I’m not familiar with it, so I’m afraid I can’t offer much help with that.
EDIT since I didn’t see your latest post: The formula considers the initial position to be at coordinates (0,0), so you will want to use the difference between the target’s location and the barrel’s.
Once you have this distance vector, project it along the up vector to get the formula’s y, and along the normalized horizontal distance to get the formula’s x.
There’s also a mistake in how you grouped the terms in the formula: v^4 - g(gx^2 + 2yv^2) should be grouped as v^4 and g(etc.), so basically multiply by g before subtracting from v^4.
Another edit: I just saw your barrel was at coordinates (0,0,0), so that will work (but the more general case of getting the difference is preferred).
Here’s a screenshot of how I got the formula’s x and y coordinates: