The original deep hole drilling problem — a straight bore 60–90× longer than its diameter, drilled through hardened steel to a straightness a twist drill can never reach. From the 5.56 mm M16 barrel to the .308 match rifle, every barrel starts as a gundrilled, reamed, and rifled deep hole in 4150 or 416R steel.
| Challenge | Why It Matters | How Deep Hole Drilling Answers It |
|---|---|---|
| Straightness | A deviating bore ruins accuracy and, worse, barrel strength | Gundrill self-pilots on two carbide guide pads that burnish and steer the bore |
| Depth ratio | Barrel bores run 60:1 to 100:1, far past twist-drill limits | Gundrilling holds straightness to 0.001 in/ft at 300:1+ |
| Bore surface | Rifling needs a smooth, uniform, correctly sized bore | Drilled undersize, then reamed to final diameter in preparation for rifling |
| Chip evacuation | Chips cannot clear a deep hole on their own | High-pressure coolant flows through the hollow drill and flushes chips out the V-shaped flute |
| Entry guidance | An off-square start drifts the whole hole | A guide bushing and pilot hole control entry geometry |
Barrel steels must balance hardness, toughness, and wear resistance. Three grades dominate: 4140 and 4150 chromium-molybdenum alloys, and 416R free-machining stainless. The choice is accuracy vs. durability.
| Grade | Carbon | Hardness | Machinability | Typical Use |
|---|---|---|---|---|
| 4140 Cr-Mo | 0.38–0.43% | ~197 HB annealed; 28–32 HRC Q&T; to ~50 HRC max | Easiest of the three — lower tool wear | Economical sport and budget barrels |
| 4150 CMV | 0.48–0.53% | ~212 HB annealed; 30–36 HRC Q&T; to ~55 HRC max | Moderate to difficult — harder, more wear on tooling | Mil-spec barrels (M4, M240), continuous fire |
| 416R Stainless | Sulfur-bearing martensitic | 26–32 HRC bar stock (24–36 range); 38–42 HRC finished match barrels | Easy — free-machining, ideal for rifling & chambers | Match and precision barrels for maximum accuracy |
Bore size is quoted either across the lands (the high points of the rifling) or across the grooves. Common centerfire calibers run from roughly 5.6 mm to 7.8 mm bullet diameter.
| Caliber | Bore (land-to-land) | Groove (bullet) diameter | Typical Barrel Length |
|---|---|---|---|
| 5.56mm / .223 | .219 in (5.6 mm) | .224 in (5.7 mm) | 10.5–20 in |
| 6.5 Creedmoor | .256 in (6.5 mm) | .264 in (6.7 mm) | 20–24 in |
| .308 Win / 7.62 | .300 in (7.6 mm) | .308 in (7.8 mm) | 12.5–18.5 in |
The gundrill is a single-lip, carbide-tipped tool with a hollow body and an external V-shaped flute. High-pressure coolant is pumped through the shank to the cutting edge, then flushes chips back along the flute. The asymmetrical cutting edge and two carbide guide pads make the tool self-piloting — the freshly cut hole guides the tool forward, which is the entire secret of barrel straightness.
Smaller drills need higher pressure. These are published starting pressures — ideal and absolute minimum.
| Drill Diameter | Ideal Pressure | Minimum |
|---|---|---|
| 0.125 in (3.2 mm) | 1500 psi | 500 psi |
| 0.250 in (6.4 mm) | 925 psi | 350 psi |
| 0.375 in (9.5 mm) | 675 psi | 300 psi |
| 0.500 in (12.7 mm) | 525 psi | 250 psi |
| 0.750 in (19 mm) | 400 psi | 175 psi |
| 1.000 in (25.4 mm) | 300 psi | 150 psi |
Gun drills are sized to produce a hole a few ten-thousandths of an inch under the final diameter. The drilled bore is then reamed to its exact size, removing the drill's tool marks and laying a uniform surface for the rifling step. Reaming typically removes on the order of 0.2–0.5 mm of stock.
Rifling cuts or forms helical grooves inside the bore that spin-stabilize the bullet. Three methods dominate — cut rifling, button rifling, and cold hammer forging — and makers disagree hotly about which is "most accurate."
| Method | How It Works | Speed / Volume | Stress to Steel | Typical Users |
|---|---|---|---|---|
| Cut rifling | Single-point carbide hook cutter removes metal one groove at a time, many passes | Slow, labor-intensive, expensive | Least stress; twist rate infinitely flexible | Krieger, benchrest and long-range makers, Pratt & Whitney-lineage hydraulic machines |
| Button rifling | Tungsten carbide button with reverse rifling impression pushed or pulled through the bore in a cold-forming single pass | Fast — about one minute per barrel; volume friendly | Significant; requires stress-relieving afterward | Savage, Faxon, Winchester, Browning, Lothar Walther |
| Cold hammer forging | Mandrel carrying the reverse rifling is inserted, hammers strike the outside of the blank | Fastest for mass production; huge capital cost | Most stress; twist fixed by the mandrel | Large military and volume manufacturers |
A precision barrel blank travels through this sequence (based on documented cut-rifled barrel production):
Annealed, normalized, and stress-relieved blanks; precision shops double stress-relieve or cryogenically treat before any cutting.
Gundrill the straight through-hole on dedicated bore-drilling equipment, guided by bushing and high-pressure coolant.
Outside profile is turned on a CNC lathe, establishing barrel taper and weight distribution.
Ream to final diameter, smoothing drill marks and preparing the surface for rifling.
Button, cut, or hammer-forge the grooves. Cut-rifled barrels are often hand-lapped between and after rifling stages; button barrels are stress-relieved after.
The chamber is reamed at the breech end and the muzzle crowned after the barrel is contoured, polished, and fluted.
Air gauging and borescope inspection verify bore diameter, straightness, and rifling integrity before the barrel ships.
| Parameter | Typical Spec | Notes |
|---|---|---|
| Straightness | 0.001–0.005 in/ft (≤0.05 mm/m) | Gundrilled bores run near 0.001 in/ft with guide-pad self-piloting |
| Diameter tolerance | ±0.001–0.003 in (IT7–IT8) | Gundrills with guide pads commonly hold ±0.0005 in |
| Surface finish | Ra 0.4–0.8 μm | After reaming and lapping |
| Bore / groove sizing | Land vs. groove diameters quoted per caliber | e.g. .223 bore .219 in, groove .224 in |
| Depth ratio | 50:1–100:1 typical; 300:1+ maximum | Single-pass gun drilling |
| Capability | Specification |
|---|---|
| Work rotation | Part rotates, drill stationary — standard on dedicated gun drilling machines; counter-rotation for best straightness |
| Coolant pump | Variable-volume, pressure-compensated; 2000 psi+ typical for deep-hole gundrilling |
| Guide bushing | Front bushing in the chip box; +0.003–0.008 in clearance; held concentric to spindle |
| Whip guides | Intermediate supports for very long drills to prevent whip and axis wander |
| Filtration | ≤10 μm particle control on the high-pressure coolant loop |
| Cycle example | 0.25 in drill, low-carbon steel: ~6,875 rpm, 3.1 IPM; larger drills run slower with heavier feeds |
Measures bore diameter along the full bore length
Visual inspection of the finished bore
Process-side quality signals