Neck-Expander Mandrels for More Uniform Neck Tension
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. Keeping your neck tension very uniform allows more consistent bullet seating. That, in turn, usually yields better accuracy, 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 Int’l and K&M will both 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. The Sinclair version, is shown above. This $24.99 unit fits caliber-specific expander mandrels ($9.95) which measure approximately .001″ less than bullet diameter for each caliber. This is an updated “Gen II” design that completely captures the mandrel within the die so the mandrel cannot pull out. It also has an O-ring in the die cap that allows the mandrel to self-center within the case neck. Sinclair now offers three sizes of die bodies for expander mandrels: .17 -.310 Caliber (#849-011-715WS); .357 – .50 caliber (#749-008-843WS), and a special .50 Cal die body for large-diameter 50 BMG presses (#749-009-163WS, $49.99). All Generation II dies are machined from stainless steel and the standard diameter 7/8-14 dies include the Sinclair Stainless Steel Split Lock Ring.
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.
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Tags: Expander Die, Mandrel, Neck Sizing, Neck Tension, Sinclair Int'l
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.
I use one of these every time I load a piece of brass new or once fired. These are also the way to go for expanding your favorite 6.5 x .284 brass to a straight .284.
One tip, The end of the expander mandrel is fairly blunt and can catch the case rim. It is much better if you round the point a bit on a lathe or even chucked up in a drill.
I used the Sinclair Expander Mandrel on once fired .338 Lapua brass after sizing. Check loaded rounds on concentricity gauge. Loaded rounds WITHOUT using sinclair expander mandrel read from .000 to .002. Loaded rounds USING mandrel all read from .003 to .005 out of round.
Cole, where were you indicating on the loaded ammo?
I only use the mandrel on new brass. My reason has less to do with concentricity, than fact that new case mouths often are slightly dinged and not completely round. I chamfer all new cases to prevent shaving of jackets. To chamfer out-of-round case mouths would result in an uneven cut. Running the cases through the mandrels prepare the cases for chamfering.
I agree that these expanders made the process of seating bullets go much more smoothly. It also reduced runout from offset setting.
The bottom line is last sizing of necks is expansion, either with a mandrel or bullets, and bullets make terrible expanders.
“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”. OK, maybe, but isn’t the degree to which the brass is worked the same whether you run a ball or a mandrel through the necks? It is still sized down by the die and up by the ball/mandrel. Also, why exactly would a mandrel give more consistent neck tension than an expander ball? I guess I’m missing something…
Editor: With many full-length sizing dies (unless they have been honed to a larger-than-standard neck diameter), you are reducing the neck diameter well below the diameter “out of the box”. Then you have to push it back out with the expander ball. Here we are talking about expanding new brass.
Thanks for the explanation. So we’re talking about hand-loading, not re-loading here. I’m good with that. Especially if those with unwanted heaps of over-worked, sub-prime, once-fired Lapua brass cluttering up their loading benches would like to clear the decks in my direction! (.308, 6.5CM, .243, .223 or .222 only please!)
This is a tad off subject. On a magnum cartridge with a muzzle brake on rifle, how many thousandths of neck tension will prevent shifting of bullets in magazine from recoil?
Should I crimp or is a tighter inside neck diameter
sufficient? This would help in order to buy proper
mandrel sizes. Thanks