Gun drilling. If you didn’t know any better, you’d think it’s meant only for firearm manufacturing. And while that’s where the process started—crafting rifle barrels with long, straight bores—don’t let the name fool you. Today, gun drilling is the go-to method for parts that require extremely deep yet accurate holes, whether for a medical implant, a plastic injection mold, or an aerospace fuel system.
Gun Drilling 101
As any machinist can attest, twist drills begin to wander once the hole gets much more than ten times its diameter deep. If that. In addition, chip evacuation becomes problematic at this depth. This can not only ruin the hole’s surface finish, but worst case, snap the drill bit clean off.
It’s good news, then, that engineers designed gun drilling for the long haul. Here, a single cutting edge does the work while a steady stream of coolant flows through the tool, flushing chips back out of the hole. The result? Holes enjoy remarkable straightness, smooth surfaces, and depth-to-diameter ratios up to 400:1. Think of it as the difference between digging a trench with a shovel versus boring a tunnel through a mountain. One tool simply isn’t made for the other’s job.
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Where It’s Used
Gun drilling has grown far beyond its firearms roots, however. Medical device manufacturers use it to drill tiny, accurate holes in surgical tools and implants. Automakers rely on gun drilling for precision fuel rails, injectors, and shafts. Moldmakers use it to drill deep cooling channels in plastic injection molds, while aerospace and defense manufacturers produce lightweight, high-strength parts with bores that are both long and exact. And the oil and gas industry depends on it for all manner of deep holes, often in difficult materials like Inconel and duplex steels. Here again, if the hole is deep, gun drilling is usually the most effective—sometimes the only—way to get it done.
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Gun Drilling vs. BTA Drilling
If you’re wondering whether gun drilling is the only option, the answer is no. Its big brother, BTA drilling (an acronym for the process developed by the Boring and Trepanning Association), works on the same principle, but machinists typically prefer it for larger diameter holes. The two differ in how the cutting fluid flows in and chips get out:
- Gun drilling: Coolant goes through the tool, and chips travel back along the single flute. Best for smaller diameters (1–50 mm).
Image Source: AGrade Carbide
- BTA drilling: Coolant surrounds the tool with chips exiting through the inside. Better for bigger holes, say those up to 75 mm in diameter.
Image Source: AGrade Carbide
Just as a shop will use different end mills and turning tools for their machining work, those that drill a lot of deep holes often keep both types of tools in their crib. For bigger bores like those in oil and gas components, BTA is king; for everything else, gun drills are usually the best way to produce high-quality deep holes. The good news is, the same machine tool can do both.
Choices, Choices
As with any machining process, which one to use depends on the workpiece material, production quantity, and level of precision needed. Gun drilling is no different. For small, relatively shallow holes, there’s nothing wrong with that old standby, the twist drill or one of its indexable cousins. But as noted earlier, machinists consider gun drilling the undisputed champion in terms of accuracy and throughput for any hole much more than 10x its diameter in depth.
If you’re exploring ways to bring that capability in-house, companies like Kent USA design machines specifically for deep hole drilling. With features like high-rigidity structures, precision spindles, and high-pressure coolant systems, their equipment gives the best bang for the buck. Some models can even drill multiple holes simultaneously. Long story short, gun drilling remains a vital part of manufacturing despite its centuries-old history. Maybe it’s time to take a new look at an old friend.
