Expander Mandrels — Not Just for Neck-Turning
Lapua brass is so good that you’ll be tempted to just load and shoot, if you have a “no-turn” chamber. However, some minimal case prep will ensure more uniform neck tension. This will produce better accuracy, more consistent bullet seating, and lower Extreme Spread and Standard Deviation (ES/SD). Lapua brass, particularly 6BR, 6.5×47, .243 Win and .308 Win comes from the factory with tighter-than-optimal necks. Before you seat bullets, at a minimum, you should inside chamfer the case mouths, after running an expander mandrel down the necks.
The expander mandrels from both Sinclair and K&M will leave the necks with enough neck tension (more than .001″) so you can then seat bullets without another operation. Put a bit of lube on the mandrel before running it down the necks — but remove any lube that gets inside the necks before seating bullets.
Both Sinclair and K&M Tools make a die body specifically to hold expander mandrels. Sinclair’s Generation II Expander Die Body (item 05-3000, shown above) completely captures the mandrel within the die so the mandrel cannot pull out. An O-ring in the die cap allows the mandrel to float a bit and find its own center within the case neck. This $24.95 unit fits caliber-specific expander mandrels (item E-XX, $8.75) which measure approximately .001″ less than bullet diameter for each caliber. Once you run the Sinclair expander mandrel down the necks of Lapua brass, after you account for brass spring-back, you’ll have about .002″ neck tension. This will make the process of seating bullets go much more smoothly, and you will also iron out any dents in the case mouths. Once the case mouths are all expanded, and uniformly round, then do your inside neck chamfering/deburring. The same expander mandrels can be used to “neck-up” smaller diameter brass, or prepare brass for neck-turning.
Forum member Mike Crawford adds: “These expanders can also reduce runout from offset seating. Prior to bullet seating, expand the sized necks to force thickness variance outward. With the Sinclair system, the necks will springback fine, and will not be pulled out of center. This leaves plenty of tension, and bullets seated more centered. I do this, even with turned necks, to get improved seating.”
Mandrels vs. Expander Balls on Decapping Rods
If you haven’t acquired an appropriate expander mandrel for your brass, but you DO have a full-length sizing die with an expander ball, this will also function to “iron out” the necks and reduce tension. However, using a die with an expander ball will work the necks more — since you first size them down, then the ball expands them up again. Typically (but not always), run-out is worse when using an expander ball vs. an expander mandrel.
Similar Posts:
- Expander Tool for New Brass
- Neck-Expander Mandrels for More Uniform Neck Tension
- Smart Reloading — Use Expander Mandrels with New Brass
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- Smarter Reloader — Use Expander Mandrels with Your New Brass
Tags: Mandrel, Neck Tension, Sinclair Int'l
The expander mandrels offered by Sinclair are .001″ and .002″ smaller than the applicable bullet diameter. After springback, even the -.001″ actually leaves you with more than .002″ neck tension and that is just too much for long range shooting. Solution… I had a custom carbide expander mandrel made that is .0002″ larger than the bullet diameter. After spring back, this results in reasonably light neck tension, but enough that you cannot move the bullet with your fingers. Using this expander and a miligram scale has resulted in SDs under 5 fps in my 6×47 lapua (running 6xc brass for large primer, 115s and RL17) – I hope one day expander mandrells are offered by Sinclair in the actual bullet diameter so we can all “really” control our neck tension.