The bullseye bubble ensures that the rifle is perfectly plumb while mounting your optics to line up with your NV so that point of impact can be regained after remounting the NV to the backbone/OIC1 , CARE should be taken in understanding the way NV’s work when performing this action as all NV’s are different and results may vary,
Scope rings/optics may need to be adjusted to meet the correct position of the mounted NV after mounting it them to the picatinny rail/ M-loc bridge so that they are in alignment with he scopes position mounted on the action of the rifle,
A zero check is recommend once correct alignment has been achieved , if +MOA rails/rings have been used on the scope the optics the angle should also be considered and potentially matching an MOA picatinny rail to suit tot the NV bridge degree angle, a zero MOA rail is recommend for best results but it is up to the user to adjust the mounted NV to suit there optics, this is not a plug and play solution and its up to the user to make there solution work with adjustments to there picatinny rail/ both on the action and NV bridge to suit as there are to many variables to consider,