Deep hole drilling is the machining of holes with a relatively large depth to diameter ratio, whereas normal drilling techniques produce holes where the depth is rarely more than five times the diameter (5 xD). In deep hole drilling the ratio may reach 150:1, and any hole deeper than ten times the diameter (10 xD) should certainly be considered a deep hole, requiring a specialized drilling technique.

Deep hole drilling can employ various machine set-ups: rotating workpiece, rotating tool, or both tool and workpiece rotating.

Whichever set-up is employed, the basic principles of drilling still apply, and the correct choice of cutting speeds and feeds are still crucial. Satisfactory chip breaking, and removing the chips from the cutting edges without damaging tool or workpiece, is essential.
Gun drilling is capable of producing smaller holes than the Single Tube System (STS), but the STS system is far more productive (4-6 times) and should always be the first choice when possible. The Ejector system is an alternative to STS when drilling smaller batch quantities as it does not require a special machine.