Целить в мишень. Целить из ружья в мишень. To aim at the target. To aim a gun at the target.

Нацелить ружье. Нацеливать ружье. To aim a gun.

Нацелить ружье. In English it would still mean the same.

Он нацелил ружье. He aimed his gun. Action complete, he's ready to shoot.

Он нацелиВАл ружье. He has been aiming his gun. Action was in progress, we don't know if he succeeded or not with aiming, we just know he's been doing it somewhere in the past.