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You can attach this directly to your scope using one of these adapters: https://www.opticsplanet.com/armasight-scope-mounting-system-for-clip-on-day-night-system.html
Yes, you can place it in front of the optic on the rail if you have enough rail space, if not you can use the appropriate adapter on this page: https://www.opticsplanet.com/armasight-scope-mounting-system-for-clip-on-day-night-system.html
Yes, this will work both as a handheld device and a rifle mounted clip-on device.
That will be very dependent on the size of the animal and atmospheric conditions you are using this unit in. Detection of a heat signature under ideal conditions can be up to 1,000+ yards so actual identification of the animal will be less. I'd say right around 500 yards you should be able to identify most animals under most conditions, but this is an estimate as I mentioned above, detection/identification is dependent on more than just the unit.
Unfortunately there is no direct adapter, you would have to mount it on a picatinny rail in front of the ACOG device. Some people use the KillFlash adapter for the ACOG with the KillFlash itself removed to allow the rubber sleeve on the eyecap to create a full light seal with the ACOG.