I use this small brush from Adams to get between the caliper and the barrel. Unfortunately, it's sold out now.
https://adamspolishes.com/products/a...ni-turbo-stick
For the rest of the barrel, I use a Woolly Wormit. It works really well but doesn't fit between the caliper and barrel on my 20" wheels.
https://www.amazon.com/Detailing-Int...e%2C105&sr=1-2