1. Preheat oven to 350°.
2. Weigh or lightly spoon brown rice flour, potato starch, and oat flour into dry measuring cups; level with a knife. Combine brown rice flour, potato starch, oat flour, granulated sugar, and next 4 ingredients (through salt) in a medium bowl; stir with a whisk. Make a well in center of mixture. Combine milk, butter, and eggs; stir with a whisk. Add to flour mixture, stirring just until moist. Fold in berries.
3. Place 12 paper muffin cup liners in muffin cups; coat liners with cooking spray. Spoon batter into prepared cups, and sprinkle with turbinado sugar. Bake at 350° for 25 minutes or until lightly browned and muffins spring back when lightly touched. Cool 10 minutes in pan on a wire rack; remove from pan.
If you're planning to freeze all or some of the muffins, leave off the turbinado sugar.