Frequently Asked Questions

  • Can gun drills be used on conventional manual or CNC machines?

    Yes - Gun drills can be used on conventional machinery with suitable coolant type and high pressure through tool coolant. Our Spraymist System is a low cost alternative to retrofit high pressure coolant delivery systems and is most practical on machinery that is not enclosed. The machine should have, or be made to have a workable, if not ideal, speed and feed capability for the hole diameter and material condition. A pilot hole to starts the gun drill instead of a guide bushing. Manual machines may require feed control modification.

  • What materials can be drilled with gun drills?

    Almost any materials can be gun drilled from ferrous and non-ferrous metals, powder metals, composites, woods, plastics, and more. Non-metals work better with gun drills ground specifically for that purpose but standard gun drill grinds will generally work bur may produce less desirable results.

  • Can I re-coat my re-sharpened and/or re-tipped gun drills?

    Coated gun drills provide an advantage in tool life and surface finish in many materials. New drills, coated, often perform better after the first sharpening, the sharper edge forming a smaller chip. Re-sharpening coated gun drills removes only clearance from the end of the drill leaving the coating where it is needed on the drill face where the cutting action occurs, and on the burnishing pads on the drill's periphery. Gun drills can be re-sharpened several times and continue to provide a tool life and finish advantage with no need to re-coat.

    If re-coating of a gun drill is desired there is an issue:

    When gun drills are coated, the coater's price depends on the type of coating, the largest diameter of the tool, and its overall length (the amount of space taken in the coating chamber). Drills previously manufactured have drivers attached. Requests to "re-sharp, or re-tip and re-coat" are priced at the largest diameter of the assembly, a 3/4" driver, for example. This is a considerably higher price than the cost of coating just the tip diameter. We can remove the driver before coating, re-braze the driver and true the assembly after coating, but this adds a comparable cost. Tip coating cost for a new, 11/32" gun drill x 10" without driver is $12 each for TiN, $18 each for TiCN, or $24 each for TiALN, and about 7-9 working days.

    For example, to re-sharp and re-coat an 11/32" x 10" gun drill with a ¾" diameter driver, the largest diameter of the assembly:



    Total Cost








    +$57.00 (cost @ 3/4" diameter


    Not a good option!
    A re-tip and re-coat gun drill would have a higher cost than a new drill, coated.

  • Where would I use a gun reamer?

    In most applications, gun drilling eliminates the need for reaming, boring, etc. The hole quality achieved by gun drilling in one pass is better than twist drilling, reaming and/or boring. Any hole quality improvement with size and finish after gun drilling, if required, would be by honing or possibly reaming with Mapal or Cogsdill type reamers.

    Single flute gun reaming can be used to correct errors made in hole sizing by opening up a gun drilled hole to a larger size, or to open up part of a bore leaving a stepped hole where concentricity between the diameters is not critical. Where concentricity is critical, a non-cut pilot on the gun drill tip will keep the tip centered as the major diameter "reams" part of the hole. Single flute reamers pilot off the tip geometry, two or more flute reamers follow the drilled hole. Chip control is problematic with a single flute gun drill used as a reamer as it produces a strand of wire the width of the cut and the thickness of the feed. This wire will then wrap up around the tip and shank and also score the hole finish. A series of pauses in the feed can lessen the problem if the machine capable of doing this.

    We offer two gun reamer styles. "Chips Back" for blind hole applications is basically a standard single flute with a different nose grind and Contour. This reamer uses through tool oil to pressurize the bore and force the chip (wire) back and out the bore. "Chips Ahead" reamers are for through hole applications and use a round (non "V" formed) tube allowing the oil to exit at the base of the tip's braze joint. The carbide tip's oil hole is plugged. Larger "chips ahead" reamers are machined at the base of the tip and tube braze joint to direct oil flow to the outer cutting corner of the tip.

    Engineers, Machinists, and machine operators alike not familiar with the gun drilling process see reaming as an expected step in the production of accurately sized holes. When results of gun drilling is observed to show what is possible in one operation with a gun drill, reaming as part of a deep or hole precision hole drilling process is no longer required.

    Reaming in gun barrel production reverts to Pacific Tool & Gauge, contact Dave Kiff. Website: A gun barrel is gun drilled to .008" to .010" under finish diameter. A PT&G multi-flute reamer, TiN coated but ground on the flute edges is either pushed or pulled through the bore lubricated by through tool oil. Surface finish and bore sizing is then complete with chambering and either cut or button rifling to follow.

    Sterling Gun Drills does not offer single flute reamers for gun barrel production. However, in 416 Stainless Steel for example, a gun drilled hole using high pressure oil would provide very good hole quality to size & finish. Gun drilling to size without reaming is not common with rifle barrels, but we do provide gun drills for pistol barrel production where the calibers are drilled to finish size. At this point, "that just how it's done" prevails.

  • Why does my gun drill measure undersize?

    The Gun Drill R-1 Lowland Contour Single flute gun drills from most manufacturers in the United States are furnished as standard with an R-1 Lowland Contour (with N-8, 30/20 nose grind). This Contour is a clear choice for a variety of applications where its free cutting action provides the gun drilling benefit of precision, burnished holes to virtually any depth. In difficult materials such as Nickel base, Coppers, Titanium, and others that can wear a drill quickly or close-up on the drill tip, clearance provided by the R-1 Contour make the process more successful. Additional benefit of the R-1 Contour are with short holes for gun drilling (under 15x dia./depth) where any inaccuracy of the drill, miss-alignment, or by the machine itself can lead to oversize holes, bell mouthing, poor finish, or other detraction of hole quality. The R-1 Contour is beneficial in many applications where operators may be unfamiliar with the gun drilling process, and/or conditions are less controlled, and/or the coolant type and pressure is not optimal. This total combination of variables make the R-1 "standard" Contour least likely to cause "issues".

    Alternate Contours such as R-2 Lowland & Top, R-3 Highland, or R-4 Highland & Top can provide benefits such as drilling more accurate holes over 50 diameters deep, self-support for cross holes, angled entry and exit, and reaming. These Contours are not covered in this writing, but each have their own place for specific applications. Refer to section Deep Hole Gun Drills, then Nose Grind-Contour Combinations for additional detail.

    To produce the R-1 Lowland Contour, the carbide tip is cylindrically ground to an industry standard diameter of +0, -.0002" (.005mm), and to a standard back taper of .0006"-.0008" (.015-.020mm) per inch. A Contour plus "back taper" is necessary as it allows just the end of the drill tip to cut & burnish and prevent the tip from "seizing" in the bore. At this point in manufacture, it is the last time the tip can be measured by micrometer. The R-1 Contour is a pattern of relief that is ground into the periphery of the tip's diameter. When ground and viewed from the tip's end, a "V" channel for oil & chip removal is approximately 115º wide. On the periphery, a "cutting land" that starts from the flat face of the drill tip is on size and about .02"-.04" (.05-1.0mm) wide, depending on drill diameter. "Relief" is ground into the tip's diameter to about .005" (.12mm) deep and extends from the cutting land to about 70º around the periphery. A full tip diameter "burnishing pad" remains from 70º to about 170º-175º from the cutting edge. The .005" relief returns for the remainder of the tip's periphery. The burnishing pad provides the fine hole finishes common with gun drilling. As this pad does not extend 180º from the cutting land, measurement by micrometer will indicate a false reading of about .001" to .002" under the gun drills true diameter. The drill is truly on size! A pilot hole or drill bushing to start the gun drill must be sized to the gun drills marked diameter +.0002" to .0005", NEVER undersize.

  • What is the maximum hardness for gun drills?

    Gun drills will produce holes on materials up to around 50Rc. The highest ranges have to run at a considerably reduced RPM to less hard materials, so drilling time is extended and holes per grind are reduced. Tip coatings such as TiAlN can improve performance at elevated hardness ranges but if at all possible, material hardness in materials up to 40Rc provide best performance.

  • What is the minimum and maximum hole depth?

    The minimum hole depth can be any hole where a superior level of precision is required. High precision holes as shallow as 3 - 5 diameters deep could be gun drilled. At the other extreme, holes as much as 100 diameters deep or more can and should be gun drilled. At these depths, especially if a blind hole, there is usually no other choice.

  • How do I define a gun drill?

    Gun drills are most often described by their overall length. This should always be confirmed when someone asks? "I need a drill 16" long" Is this the drill depth or overall length? A 16" overall length gun drill with a standard 2 ¾" long driver will drill about 12" to 12.5" deep on manual or CNC conventional machinery. The extra ¾" to 1 ¼" allows for chip clearance and re-sharpening.

  • What coolant should I use?

    Gundrilling oil or its equivalent is the best for tool life and finish. If a water soluble is used, it should be high "EP", oil based, at 10% concentration. A synthetic coolant should never be used. Semi-synthetics are generally not acceptable and may lead to premature tool wear and failure. Vegetable based coolants that are reported to have very good lubricating qualities and may also be suitable in the certain applications.

  • How do I start a gun drill?

    On CNC or Manual machines, a pilot hole is usually easier to produce than to fixture for a bushing start similar to a gundrilling machine. The pilot hole should be sized to the gun drill diameter +.0005" to .001", but NEVER undersize. NOTE: On rotating drill applications, the gun drill should NEVER rotate outside of the hole or bushing. The carbide tip is much heavier than the tube and the unbalanced geometry of the assembly will cause the drill to whip and likely break off; extremely dangerous for personnel within range.

  • What is Speed & Feed?

    Elsewhere on this site we have a downloadable Speed & Feed Chart. Speed & feed is dependent on coolant type, pressure, material type and condition, hole depth and drill style. Speed & feeds are provided as a conservative starting point and will be affected by all operating conditions.

    See: Speed & Feed Charts.

  • Do I need non-standard or special grinds for specific materials?

    Generally NO, with exceptions for drilling conditions such as cross holes, angled entry and/or exit, or very deep holes. It is common for competitors to specify non-standard grinds for various materials when a standard grind will do. Special grinds by some manufacturers can be a method of making gun drill features unique so it adds complication to quotes from competitors. Or sometimes, special features are unnecessarily specified simply as a means to convince unsuspecting customers of technical ability. Most important is correct coolant type and pressure along with suitable speed & feed.

  • What can I expect for tool life?

    The hardest question to accurately answer. Tool life depends on every condition of the operation. The nearer to optimal conditions of oil type, pressure, speed & feed the better the tool life. Generally, tool life between re-grinds has to be determined by the drill in use, either by linear inches, minutes, parts drilled, or whatever tracking method one chooses and then sticks with it.

  • How do I support a very long drill?

    On gundrilling machines, intermediate supports with anti-whip guides handle this job. On manual and CNC conventional machinery, holes beyond 30-50 diameters deep should be supported with an intermediate flute support(s), or by use of intermediate length drill(s) enabling the drilled hole to act as a flute support for the next longer drill.

  • How accurate is a gun drill?

    .0005"/inch or better straightness is common with an accurate set-up and proper operating conditions. Best accuracy is achieved with counter rotating work and drill.

  • Why would I choose a gun drill over a coolant feed drill?

    The main criteria is accuracy. The hole size, finish, straightness, are all far better with a gun drill than twist drilling. If there is any second, third, or more finishing operations to achieve the required accuracy, then gun drilling is preferred. If the hole is simply for clearance with speed more important than precision, the twist drill may still be the best.

More information about Gun Drill Styles & their uses?