Fully Understanding Taper Crimp in Reloading

Introduction

Taper crimp is a critical yet often misunderstood aspect of reloading, particularly for novice reloaders. This guide aims to provide a comprehensive, single-source reference on the subject, drawing from extensive testing and practical experience. The author(s), one, a seasoned 9mm Luger ammunition reloader since 1978 and mechanical engineer with expertise in tolerance analysis and metrology and the other. also an avid reloader since 1980 and a degreed physicist, have compiled this information to dispel myths and misinformation.

What Taper Crimp Is Not

• Bullet Retention: Taper crimp does not hold the bullet in place. Modern auto cartridges rely on “case mouth tension” to retain the bullet. If the bullet recedes into the case with moderate pressure, the issue lies with the bullet’s outside diameter (OD) being too small or the case mouth’s internal diameter (ID) being too large.
• Tightening the Bullet: More taper crimp does not necessarily hold the bullet tighter. Excessive crimp can distort the case, loosening the grip on the bullet. The effectiveness of taper crimp can also vary based on the brand of crimp die and brass case used.

What Taper Crimp Is

• Case Mouth Preparation: Taper crimp is the process of removing the “belling” or “flaring” of the case mouth after the bullet is seated. This ensures the case mouth returns to its pre-flare shape, facilitating smooth feeding and chambering.
• Diagnosis in Straight-Walled Cartridges: Cartridges like 45ACP, 38 Super, 380 Auto, and 40S&W have straight walls. Any flare or belling on these cartridges will prevent them from entering the chamber, making diagnosis straightforward.
• Diagnosis in Tapered Cartridges: The 9x19mm Luger cartridge, designed in 1902, has a tapered exterior. Any flare or belling can cause the cartridge to partially enter the chamber and stop, making diagnosis more challenging. Measurements for taper crimp should be taken on the last 1mm (.040″) of the case mouth.

Optimal Taper Crimp Dimensions

• Mathematical Calculation: The optimal taper crimp diameter is calculated as 1 bullet diameter plus 2 case mouth thicknesses. For 9mm cartridges with plated or jacketed bullets, this measurement typically falls between 0.375 and 0.378″.
• Practical Measurement: A practical method is to press the bullet into a sized, un-belled case and measure the outcome. This method is effective for consistent bullet and brass combinations but may vary with different brands.

Practical Taper Crimp Dimensions

• SAAMI Dimensions: The dimensions provided in SAAMI cartridge drawings are generally the maximum permissible dimensions. For reliable feeding in auto-loading pistols, a clearance of 0.002-0.003″ is recommended.
• Useful Taper Crimp Range: For 9×19 Luger, a useful taper crimp diameter is 0.376-0.379″, with most reloaders using 0.377-0.378″. This range ensures good feeding and slide closure.
• Barrel Dependency: Taper crimp is a function of the barrel, as it affects cartridge feeding. Once set, the taper crimp diameter should not change unless the barrel is changed.

Setting the Taper Crimp Die

• Test Cartridges: Create 5 to 8 test cartridges without primer and powder. Use these to adjust the taper crimp die by lowering it in small increments until the desired dimension is achieved.
• Die Adjustment: Due to the “spring back” quality of brass, cases cannot change less than 0.003-0.004″ in a single crimp. Adjustments should be checked during production and annually to account for die wear.
• Brand and Length Variability: Different brands of brass and case lengths can result in slight variations in taper crimp diameter. Aim for an average case mouth diameter in the 0.377 to 0.378″ range.

Testing Taper Crimp

• Barrel as Gauge: The barrel is the ultimate gauge for cartridge dimensions. Cartridges should fall fully into the chamber using their own weight, fully headspace, and fall back out. A pleasing “Tink!” sound indicates proper sizing.
• Clearance Check: Twist the cartridge while headspaced to check for adequate clearance. If it does not spin freely, the bullet’s ogive may be bound in the rifling, creating a dangerous situation.

Recommended Dies

• Lee Factory Crimp Die: The Lee one-piece die (product number 90780) is affordable