This article is part of the CNC Machining Processes Guide: Milling, Turning, EDM & Multi-Axis on CNX Precision.
Gun drilling is a single-flute deep-hole drilling process built to produce very deep, straight, and accurate holes at high length-to-diameter ratios. Where a conventional twist drill wanders after a few diameters of depth, gun drilling holds alignment and a fine surface finish tens of diameters into the workpiece. The name traces back to firearm barrels, the first major application, but today the process serves the hydraulic, energy, mold, and precision shaft industries. CNX Precision applies this capability to deep-hole components for B2B export customers worldwide. This guide explains how the process works, what tolerances you can expect, and how to prepare your drawings for production.
How Gun Drilling Works
A gun drill is a single-flute tool with a V-shaped flute that runs along a tubular shank, usually made of carbide or built around a carbide tip. High-pressure coolant travels through an internal channel in the drill, reaches the cutting edge, cools the cut, and flushes chips up the V-flute and out of the hole. Because the chips evacuate immediately and never recut, the bore stays clean and dimensionally stable even at extreme depths.
The process begins with a controlled entry. The drill tip starts in a hardened starting bushing or a short pilot hole that aligns the cutting edge for the first few millimeters. Once inside, polished carbide pads on the drill circumference ride against the hole wall. These pads guide the tool, damp vibration, and burnish the surface as the drill advances. That self-piloting action is the reason gun drilling can hold straightness over depths that other methods cannot reach.
Modern gun drills use brazed or indexable carbide tips, and guide pads can often be replaced independently. Tool geometry is matched to the workpiece material: tougher substrates and sharper edges suit stainless and heat-resistant alloys, while harder grades suit cast iron and hardened steels. This flexibility lets one process serve a wide range of part materials.
Achievable Depth, Straightness, and Surface Finish
Gun drilling routinely handles length-to-diameter ratios of 20:1 to 100:1, and capable shops push beyond that when the application demands it. Common working diameters run from roughly 1 mm to 40 mm, and specialized equipment extends both ends of the range. Straightness of about 0.0005 in/in, roughly 0.05 mm per 100 mm, is a realistic production target, and diameter tolerances within a few hundredths of a millimeter are standard for most jobs.
Surface finish benefits from the same guiding action. The carbide pads polish the bore wall as the drill passes, so finishes of 1 to 3 microns Ra are typical in a single pass. Where a finer finish is required, secondary operations such as honing, skiving, or roller burnishing can follow. Because the hole is straight and round from the start, these finishing steps remove less material and cost less overall. Roundness and surface integrity matter most where a bore carries a seal or a sliding spool, because a smooth, straight bore reduces wear and leakage in service.
Setup and Machine Requirements
Successful deep-hole work begins with rigidity and alignment. The workpiece must be fixtured so the drill enters square to the surface, and a starting bushing or precision pilot keeps the tip on axis during entry. The spindle, bushing, and workpiece must all share the same centerline, because machine geometry directly controls hole quality. Dedicated machines come in horizontal single- and multi-spindle configurations, and some shops also run the process on machining centers fitted with high-pressure coolant and support bushings.
Coolant is the second critical element. A high-pressure pump delivers filtered oil or emulsion through the drill at rates and pressures matched to the diameter and material. The system must carry the full chip volume without interrupting flow, because even a short loss of coolant can stall chip evacuation. Long workpieces usually need steady rests or a supporting counter-guide, and feeds and speeds must suit the material. Pecking is generally unnecessary, since chips clear continuously through the flute.
Applications Across Industries
The process still serves the firearms market that gave it its name, but today it appears wherever long, precise bores matter. Hydraulic manifolds and valve bodies rely on deep intersecting passages, and spool valves need bores with tight straightness and finish. In oil and gas, the method machines valve bodies, downhole tools, and instrumentation parts that see sustained pressure. Mold makers use deep holes for cooling channels and ejector pin holes, which improves cycle times and part quality. Automotive, aerospace, and power generation apply it to shafts, rollers, spindles, and fuel system components, and medical manufacturers use small deep holes in instruments and tooling. Any part with a bore deeper than about ten diameters is a candidate.
Design Guidelines for Deep-Hole Parts
Good deep-hole parts start on the drawing. Keep the entry face perpendicular to the hole axis whenever possible, because angled entries push the tip off center and shorten tool life. Where the hole exits, plan for breakthrough into a flat or a relieved surface, since exiting through an uneven face can deflect the drill. Avoid cross-drilled interruptions along the bore unless they are truly needed, and tell your shop about them in advance so the sequence can be planned.
Specify straightness, finish, and tolerance only where function requires them, because tightening every call-out raises cost without adding value. Consider whether one deep hole can replace several shorter drilled passages joined later, since a single straight bore is often cheaper and more reliable than complex internal galleries. Finally, think about the whole part. Deep holes often interact with later operations such as cross-drilling, threading, or honing, and sequencing those steps correctly prevents a finished bore from being distorted by nearby machining. Your supplier can review the model and suggest changes before cutting begins.
Frequently Asked Questions
What is the difference between gun drilling and BTA drilling?
Both are deep-hole methods, but they move coolant and chips in opposite directions. In gun drilling, coolant flows down an internal channel and chips exit along the outside flute. In BTA drilling, coolant flows down the outside of the drill tube and chips are pushed up through the center of the tube. BTA suits larger diameters and high-volume production, while the gun drilling approach covers a wider small-diameter range with simpler tooling.
What is the maximum depth-to-diameter ratio for gun drilling?
There is no universal limit. The practical ratio depends on diameter, material, machine stiffness, and coolant capacity. Ratios of 20:1 to 100:1 are common in production, and ratios beyond 100:1 are possible on dedicated equipment with careful tuning. For most commercial parts, the real question is how to balance cycle time, tooling, and tolerance in the most economical way.
Can gun drilling machine stainless steel and hardened steels?
Yes. With carbide tooling, suitable coatings, and the right feeds, speeds, and coolant, the process works across carbon steels, alloy steels, stainless steels, and many high-temperature alloys. Hardened materials may call for adjusted parameters or a secondary finishing pass. Sharing the material grade and hardness with your supplier allows the process to be qualified before production runs.
For related information, see our guide to cnc drilling, and deep hole drilling.