He NEEDED to let it be known that the rifle was left where anyone, allegedly wishing to use it to shoot the family, would have easy access to it.
Of course, he had to make it look easy. The difficulty for him though was that the .22 rifle was problematic and was incapable of killing using one bullet even at point blank range, the bullets just don't have sufficient velocity. The killer had to fire multiple shots to put his victim down requiring reloading at least once. None of this was in Bamber's plan. His need to reload meant Nevill could get to the kitchen where the fight ensued, a fight which Sheila could never have participated in.
The logic in this case is easy to see, the silencer issue is merely a distraction.