I'm pretty sure Barney says this in 3 episodes where he's gets so excited about something Andy is going to do for Opie and the boys. I remember for sure during Andy's "shot heard around the world speech" and forming the Mayberry Minutemen.
He also says it in Opie's Forture when Opie wonders if he should buy an erector set, and Andy tells about building a drawbridge over a stream with boats and everything, and Opie says "you think I could build something like that?" And Barn says "will you help us?"
But I think there is one other time....can you remember, or am I mistaken?