A special rifle scope or scope add on for behind cover or in trenches

 A rifle scope or sniper scope. Which has multiple parts or a part which can be fitted on.

A scope or an extra part for the scope, can be done on.

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Like, so it's possible to see through the scope, while the gun is on the cover, but

the person is hiding, not being visable, while it's possible to see through the scope

and shoot down enemies.

I've actually seen similar ideas from the WWI and WWII trenches. But they were more primitive.