Deep Hole Drilling for Bullet Swage Die Core Pins and Ammunition Cartridge Case Drawing Punches: Sporting Ammunition Manufacturing

A manufacturer of match-grade rifle bullets gun-drilled cooling channels in tungsten carbide swage die core pins (1.0 mm pin diameter, 0.3 mm bore x 120 mm length, 400:1 L/D) using a PCD-tipped micro gun drill (0.3 mm, Vc = 20 m/min, f = 0.0005 mm/rev, oil coolant at 120 bar). Concentricity was held within 0.01 mm TIR relative to the pin OD. The cooling channel reduced die surface temperature by 50 C, eliminating jacket material galling and extending die life from 50 000 to 500 000 rounds.

Bullet Swage Die Core Pin Cooling Channel Micro Drilling ​

Bullet swage die core pin cooling channels represent one of the most extreme deep hole drilling applications in precision manufacturing. A 0.3-0.5 mm diameter channel must be drilled through a 100-200 mm long tungsten carbide or tool steel core pin, achieving depth-to-diameter ratios of 200:1 to 400:1. The core pin is the central mandrel that forms the inside of the bullet jacket during swaging, where 100-150 tonnes of force (for a .308 calibre bullet) compresses the jacket material (gilding copper or cupro-nickel) between the core pin and outer die cavity. The friction generates sufficient heat to cause galling if the interface temperature exceeds approximately 200 C. The cooling channel circulates coolant (oil or water at 10-50 bar) through the centre of the pin, maintaining surface temperature below 150 C, eliminating galling, and extending die life by an order of magnitude.

Cartridge Case Drawing Punch and Flash Hole Piercing Pin Drilling ​

Cartridge case drawing punch coolant bores are drilled in H13 or M2 tool steel drawing punches that form the brass cup into the cartridge case through progressive drawing dies. The coolant bore (2-6 mm diameter x 50-150 mm depth) is drilled from the punch base through the body, ending within 5-10 mm of the punch tip. The bore provides a passage for oil coolant that lubricates the punch-cup interface and removes heat generated during the drawing process. Without the coolant bore, the punch tip temperature can exceed 300 C during high-speed production (60-120 strokes per minute), causing the brass to anneal and stick to the punch. Flash hole piercing pin bores are micro-drilled in spring steel or M2 tool steel pins that pierce the flash hole in the cartridge case web. The pin bore (0.2-0.5 mm diameter) must be concentric with the pin OD within 0.01 mm TIR to ensure the flash hole is centred in the case web.

Primer Cup and Shotshell Wad Tooling Drilling ​

Primer cup pocket forming punches require precision coolant or ejector pin bores drilled in D2 tool steel (58-62 HRC). The punch forms the primer cup pocket in the cartridge case head, and the ejector pin bore (1-3 mm diameter, 30-80 mm length) allows a spring-loaded pin to eject the formed case from the punch after each stroke. The bore must be straight and smooth (Ra < 1.6 microns) to allow the ejector pin to slide freely without binding. Shotshell wad column tooling requires gun-drilled bores in O1 tool steel for the wad pressure-forming punches that compress the fibre or plastic wad column in a shotshell. The bores are 2-8 mm diameter, 40-120 mm deep, providing passages for compressed air to eject the formed wad from the punch cavity after each cycle, maintaining production rates of 100-300 wads per minute.

FAQ ​

What peck drilling cycle is required for micro gun drilling in tungsten carbide? ​

Tungsten carbide micro drilling requires a very short peck depth of 0.5-1.0 mm per peck, due to the extremely high abrasive wear on the drill and the difficulty of evacuating fine tungsten carbide chips. At 400:1 L/D, a 120 mm deep hole requires 120-240 pecks. After each peck, the drill retracts fully from the hole to allow the coolant jet (oil at 100-150 bar) to flush the chips out of the bore. The retract speed should be high (500-1000 mm/min) to minimise the time the drill spends in the hole, while the plunge speed should be slow (5-20 mm/min) to prevent drill deflection at the start of each peck. The peck depth is the single most important process parameter for avoiding drill breakage at extreme L/D ratios -- if the peck depth exceeds 1.0 mm, the chip packing in the flute flutes causes the drill to jam and break.

How is the cooling channel concentricity verified in a 1.0 mm OD core pin? ​

Concentricity of a 0.3 mm cooling channel within a 1.0 mm OD core pin (wall thickness 0.35 mm on each side) is verified by optical microscopy. The core pin is mounted in a V-block on an optical comparator or measuring microscope (50-100x magnification). The pin OD is measured at both ends to establish the reference centre, then the cooling channel bore is imaged at both ends. The distance from the bore edge to the pin OD is measured at four points (0, 90, 180, 270 degrees) and the maximum difference is the TIR. For a 0.01 mm TIR specification, the bore centre must be within 0.005 mm of the pin centre. If the concentricity is out of specification, the pin is rejected, as there is no practical method to correct a misaligned micro bore in a fully hardened pin. X-ray inspection can also be used for verification without sectioning the pin.

What causes the bullet jacket to gall on the core pin surface? ​

Galling occurs when the interface temperature at the jacket-core pin contact exceeds approximately 200 C. At this temperature, the copper alloy jacket material (gilding copper, 90% Cu / 10% Zn) softens significantly (yield strength drops from 300 MPa at room temperature to approximately 100 MPa at 200 C). The softened copper adheres to the carbide core pin surface through adhesive wear -- microscopic asperities on the core pin surface weld to the copper jacket under the 100-150 tonne swaging pressure. As the jacket is ejected from the die, the welded material tears, leaving copper adhered to the core pin. On the next cycle, the adhered copper creates a surface defect on the jacket interior, affecting the bullet's mass distribution and accuracy. The cooling channel prevents this by maintaining the core pin surface below 150 C, keeping the copper yield strength above 200 MPa and preventing adhesive wear.

What surface finish is required inside a drawing punch coolant bore? ​

The coolant bore surface finish should be Ra < 1.6 microns. A rougher surface (Ra > 3.2 microns) creates turbulence in the coolant flow, increasing the pressure drop along the bore and reducing the coolant flow rate at the punch tip. The minimum flow rate for effective cooling is 2-5 litres/min, which requires a bore diameter of at least 2 mm for the typical 50-80 mm depth. The bore should also be free of step changes in diameter, as each step creates a pressure drop that reduces the tip flow. The coolant bore is typically gun-drilled in a single pass (without pecking for L/D < 50:1), and the surface finish is determined by the gun drill edge condition. A worn gun drill produces a rougher bore surface and should be replaced when the surface finish exceeds Ra 1.6 microns.

What heat treatment sequence is used for H13 drawing punches with coolant bores? ​

The recommended sequence is: (1) Rough machine the punch exterior, (2) Gun drill the coolant bore in the annealed condition (H13 annealed to 200-240 HB for optimal machinability), (3) Heat treat: austenitise at 1020-1050 C, quench in forced air or oil (depending on section thickness), triple temper at 540-560 C to achieve 48-52 HRC, (4) Finish grind the exterior to final dimensions. Drilling the coolant bore before heat treatment is essential because H13 undergoes approximately 0.1-0.2% dimensional change during hardening (the bore diameter shrinks by 0.002-0.010 mm). If the bore were drilled after heat treatment in the hardened condition (48-52 HRC), the gun drill life would be dramatically reduced (50-200 holes compared to 5000-20000 holes in annealed H13), and the drilling cost would increase 10-20x.

Data are based on published research and industry experience as of 2026. Always consult your equipment manufacturer and applicable ammunition industry standards (SAAMI, CIP, NATO DBL) for specific application requirements.