First we need a definition of "aim" that is interesting. If the gun had a laser sight, you would just wiggle until the red spot falls on the target (assuming, of course, that the laser also travels in a spiral). But that is just going in circles :)
So let's say that we are given 3D coordinates for gun and target, and our goal is to compute a direction vector such that when the gun is 'aimed' in that direction and fired, the target is hit. Thus, we have to determine the shot's axis; the line $L$ through the center of the spiral. But we also have to account for the initial direction of the ray: the muzzle can be placed on any point around a circle of radius $r$ orthogonal to $L$. It seems that "aiming" is more difficult due to our having one more parameter to determine. One would have to find the relative direction $\theta$ from $L$ to the target as seen from the gun, and then determine the initial position of the muzzle, given the spiral parameters and the distance.
However, the line $L$ is not unique. In fact, the is a circle's worth of lines around the target all of which give hits, provided the initial direction is chosen correctly. Once you work out the formula (which I won't do since this is just a conceptual discussion) you can hit the target in infinitely many different skew directions!
In any case, I would say that it is indeed more difficult to aim a spiral ray gun because the formula is more complicated than a straight line equation. This is an anticlimactic answer, but after trying to set up a reasonable model, it is the answer that matches my intuition.