Heck Tate was going to tell Jem that Atticus was a crack shot, but Atticus stopped him.
When Atticus shoots the mad dog, Jem and Scout are impressed. Jem starts to ask a question, but he is shocked. He had no idea his father could shoot so well.
"What's the matter with you, boy, can't you talk?" said Mr. Tate, grinning at Jem. "Didn't you know your daddy's-"
"Hush, Heck," said Atticus, "let's go back to town." (ch 10)
Atticus does not want Heck to praise his shooting abilities. He does not want his children to care about shooting, and he wants them to focus on moral courage rather than the type that requires a gun.
Later, Miss Maudie tells them “Atticus Finch was the deadest shot in Maycomb County in his time” and they call him “One-shot” because he can get his target in one shot (ch 10). Even though Atticus had not shot a gun in years, Heck Tate asked him to shoot the rabid dog because the dog was too far away for Heck to make in one shot.
At a time when the children are not satisfied with Atticus, they find out a new side of him. It demonstrates to them that he has tricks up his sleeve, and helps them develop new respect for him. It also shows his importance to the community.