Barrett has a specific list of compatible scopes, and even then, those scopes have to be set up a certain way.
http://www.barrett.net/optics/bors
Compatible Scopes
Leupold® Mark 4® LR/T® M1 3.5-10x40mm
Leupold Mark 4 LR/T M1 4.5-14x40mm & 50mm
Leupold Mark 4 LR/T M1 6.5-20x50mm
Leupold Mark 4 LR/T M1 8.5-25x50mm
Leupold Mark 4 ER/T M1 4.5-14x50mm
Leupold Mark 4 ER/T M1 6.5-20x50mm
Leupold Mark 4 ER/T M1 8.5-25x50mm
Nightforce™ NXS 3.5-15x50mm & 56mm*
Nightforce NXS 5.5-22x50mm & 56mm*
Nightforce NXS 8-32x50mm & 56mm*
Schmidt & Bender 3–12x50mm Police Marksman II LP**
Schmidt & Bender 5-25x56mm Police Marksman II LP**
*Not compatible with Zero-Stop or .1MRAD (metric) turret systems.
**Must have “double-turn” turrets with .1MRAD (1cm) adjustments.