The plan at the time was what the film explicitly said. They were devastatingly effective at taking the Tantive IV. They were devastatingly effective at destroying the sandcrawler (to the point that they couldn’t disguise it as having been attacked by sand people). They were devastatingly effective at nearly hitting the protagonists enough times that they believed they were being shot at for real, when actually they were just being herded onto a ship with a tracker on it so they’d reveal the location of the rebel base they’d spend the first half the film failing to threaten or torture out of Leia. They were devastatingly effective at destroying the rebel’s attack force against the Death Star, and were seconds away from destroying Yavin IV and the rebel base on it when someone used supposedly-extinct space magic to make what the film had previously described as an impossible shot against a tiny target.

The only time they miss is when the film says they did so on purpose.