Forget control rig and look at old tutorials.
You need an inverse IK setup to do proper aiming.
Aimoffsets can work, but you would have to define the poses mathematically for them to be only somewhat accurate looking.
Thing is, most fps are less than somewhat accurate, so maybe thats OK too.
If you go that route, simply pose the mesh in a DCC - in first person view, with an IK solver, while aiming/holding the barrel where you want it to aim based on the max/min values of the offset you want (Normally up/down 0 to 60, left/right 0 to 90 plus diagonals).
You have to make sure your camera FOV is the maximum value your project’s will ever be.
In theory, you can then shorten the values based on the fov so as to make sure that the uppr right corner is the upper right, and not off screen.
After that, remeber you can’t acrually shoot out of the barrel. Its an approximation. Your weapon needs to fire from the chamber to the reticle. Usually its just a line trace even on AAA titles anyway.
If you wanted to do it properly, then youd do the IK in unreal, and adjust your bone position based on the final aim point.
The arm chains are 3 bones each, and you really do get unnatural wrist movement if you don’t set it right. Thats particularly disturbing in first person as the wrist bones often look broken, so be mindful of the setup.