Having the correct size hole prior to threading for a bolt is critical; the incorrect size can ruin the project. There are a few variables to consider before cutting your hole. Careful planning will ensure success.
Target and Range
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Most drill charts recommend a 5/16 drill for a 3/8-16 bolt. Cutting holes that are smaller will leave more material for the tap to cut out, risking tap breakage. Drilling larger holes will result in weaker threads, increasing the possibility of stripping them.
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Material
The material you are cutting also helps determine the size of hole you need. Soft or thin materials require a smaller hole for stronger threads. Hard, tough materials and deep holes require a larger hole to ease pressure on the tap while it is cutting.
Drilling Method
Use a drill press whenever possible for greatest accuracy. If you must use a handheld power drill, start with a smaller drill bit and support the project and your arms rigidly. Slight movements of the tool while cutting the hole will cause the finished hole to be too big or, worse, could cause drill breakage.