Expander mandrel bullet tension - K&M Precision
Neck tension is one of the factors that directly affects your ES and SD. Too much tension and the bullet releases at a higher pressure, too little and it releases at a lower pressure. Variation in neck tension means variation in muzzle velocity. The expanding mandrel from K&M Precision gives you control over that variable by bringing the inside of the case neck to a precise dimension.
What the mandrel does
The mandrel is pushed through the case neck via your reloading press. The hardened steel irons the inner wall smooth and expands the neck to an exact predetermined diameter. The result is a neck that is perfectly cylindrical on the inside, not oval, not tapered, not uneven as it can be after sizing with a standard expander ball. That difference translates directly into more uniform neck tension when seating.
How neck tension works
K&M grinds the mandrels oversize to account for brass springback. After expanding, the brass springs back slightly, and the mandrel is ground so that after springback you end up at the desired neck tension. Each calibre and each neck tension has its own mandrel. They are laser etched with calibre and size so you do not mix them up.
The process is as follows. Size your cases with a full-length die (expander ball removed) or a bushing die. Bring the inside of the neck 0.002" to 0.003" smaller than your desired final size. Then run the mandrel through to bring the neck to its final dimension. Always use a light dry neck lubricant, such as Imperial Application Media or Imperial Dry Neck Lube.
0.001" or 0.002" neck tension
We stock the mandrels in two neck tension sizes: 0.001" and 0.002". The difference is how tightly the case neck grips the bullet. At 0.001" (one thousandth of an inch) the bullet sits more loosely in the neck. Seating force is lower, the bullet releases more easily. That can be beneficial in benchrest and F-Class where you seek maximum consistency at low pressures. At 0.002" (two thousandths of an inch) the bullet sits tighter. More grip, higher seating force, and slightly more resistance before the bullet releases. That is the standard choice for most precision rifle applications and PRS.
Which one you choose depends on your calibre, your ammunition application, and your preference. Not sure? Start with 0.002", that is the most versatile option.
What you need
This product is the mandrel only. To use it you need a holder that fits your reloading press. The K&M PowerStroke mandrel holder has a 7/8-14 thread and fits any standard reloading press. If you do not have a holder yet, consider the complete set with mandrel and holder together.
If you reload multiple calibres, you need a separate mandrel for each calibre but can use the same holder. Swapping mandrels takes seconds.
Specifications
• Type: expanding mandrel for bullet tension / neck tension
• Model: PowerStroke (without stop screw)
• Available neck tension: 0.001" and 0.002"
• Material: hardened steel, custom ground
• Tolerance: 0.0005"
• Marking: laser etched with calibre and neck tension
• Compensation: oversize for brass springback
• Requires: PowerStroke mandrel holder (sold separately)
• Fits: 7/8-14 reloading press via mandrel holder
Tips
Anneal your brass consistently. Springback varies with the hardness of the brass, and if your cases are unevenly annealed you will get uneven results, even with a mandrel. Always use dry neck lubrication. Without lube you pull the mandrel too tightly through the neck and damage the brass. Check your result with a gage pin if you have one. The pin should glide through the neck with light resistance, like a piston compressing air when you cover the primer pocket with your thumb.
Questions about which mandrel size you need for your calibre and desired neck tension? Get in touch, we will help you make the right choice.
