When it comes to choosing size of nails it comes down to the project you'll be tackling. For trim where you'll be hiding the head of the nail, an 18-gauage nail works in most applications unless the trim is unusually thick or heavy. Which is where a 15 or 16-gauge nail gun works best to supply additional holding power. On the other hand, a 23-gauage pin nail is best for small projects where there is little structural integrity required. Lastly, a framing nail gun is best for any sheeting or dimensional lumber projects.