It's worth remembering when using laser collimators that they themselves need collimated once in a while to ensure that the laser is aligned correctly along the axis of the barrel.
Sit the collimator in a set of V blocks and put a piece of paper on the wall on the other side of the room marking on it where the laser hits. Rotate the collimator in the V-blocks and check that the dot stays on the mark on the paper. If it doesn't then you need to adjust the collimator collimation screws until it can be rotated without the dot moving off the mark.
Hope that also helps
Cheers
Paul