I want to make a perspective like FPS game in AR with THREE.JS

I tried this way

gun.quaternion.copy(camera.quaternion);

but it didn't work.

and I tried

var pLocal = new THREE.Vector3( 0, 0, -2 );
var target = pLocal.applyMatrix4( camera.matrixWorld );
                            
gun.position.copy( target );
gun.lookAt(camera.position);

but it didn't too.

When I rotate camera, object doesn't follow and doesn't rotate.

How can I make a perspective like FPS game in AR?