From Halopedia, the Halo wiki
A Sentinel sight is a scoped weapon attachment, with variable-zoom 4×/6× optics; optimized for long-range, pinpoint lethality. Equipping one allows the user to switch between nearer and farther zoom ranges, but has greater glare on the lens than the Longshot sight.
Known Applicable weapons
The following weapons have been seen to utilize this attachment: