Vortex VMX-3T Magnifier with Flip Mount

From Vortex:
Simple, fast, effective—the push-button design of the VMX-3T engages and disengages the flip-mount, allowing the magnifier to lock in place. The result, 3x magnified or unmagnified views at will. Optics are fully multi-coated for optimal light transmission. Lightweight and tough; hard-coat-anodized machined-aluminum construction ensures durability. Internally nitrogen purged for reliable waterproof/fogproof performance. Ultimate magnified versatility for virtually any AR-height red dot sight.
Allows for lower 1/3 or absolute co-witness mounting heights and is ideal for shooters who want to increase the effective range of their red dot sight.