Have you checked if the bulb is the correct one? Presuming you got the lamp second hand??
Logun bulbs are a certain degree of light angle throw 12 degree i think?, b&q do a similar replacement bulb but these cast a shadow or halo.
Try put tape on the inside of the reflective cone of the lamp, just 5mm ish from the end, that will dampen the glow a little, however it will also reduce your lamps range a tad.
A lightforce is overkill for an air rifle unless its got a dimmer kit