KM Custom Undersized Expander Mandrels for 6mm


Price is for 1 mandrel. Click on Select Option arrow.

SKU: C1067 Category: Tag:


KM Custom Grind Machined Expander Mandrels – for controlling case neck tension

 P/N 24-0010 BT = result approximately. The actual size of this expander is approx 0.2430″
 P/N 24-0015 BT = result approximately. The actual size of this expander is approx 0.2425″
 P/N 24-0020 BT = result approximately. The actual size of this expander is approx 0.2420″
 P/N 24-0025 BT = result approximately. The actual size of this expander is approx 0.2415″
 P/N 24-0030 BT = result approximately. The actual size of this expander is approx 0.2410″
NB: KM are changing slightly the expanders which they are now supplying. New versions DO NOT have the threaded adjustment rod. They call these the “powerstroke expanders” just to identify them differently. The only difference between them is the threaded rod or not. But with any expander having a threaded rod (as per photos) you can unscrew and remove it. Therefore it becomes the same as a non threaded unit. For best height adjustments use either of the two new V2 Powerstroke press adapters (two heights available).
* Therefore, a quote from the manufacturers website…. “some sizes may still have the threaded hole as we deplete inventory”
  • These are special ground expander mandrels to be used for obtaining more precise bullet seating tension. “BT” stands for bullet tension (But please read important notes below)
  • These are custom undersize expanders to help reloaders control neck tension better. Particularly when you are not doing outside neck turning of brass which uniforms wall thickness.
  • These custom expanders are available in 1/2 thousandths of an inch increments of BULLET TENSION being the size under (or less than) the calibre size.
  • It is important to remember these are made and marked to take into account brass springback. So the size and the part numbers stamped on them refer to “bullet tension RESULT”. Not the actual measured size of the mandrels themselves!
  • For example -0.0025″ of Bullet Tension (BT) on these KM expanders means to achieve a bullet grip on neck tension of 0.0025″ and after using that expander, the brass should normally spring back to leave the internal neck size of around 0.2405″. This is why they are marked and identified this way. This is an assumption by KM based on “normal” brass springback. This also means, if using a neck bushing style die you should use a smaller bush than the regular size recommendation.
  • We at BRT do know that brass springback can vary too. So if you are aware of this too and want to know what the actual measured sizing of these expanders is, I have listed that a chart below for you. Therefore you can correlate this information to the actual part numbers listed.
  • KM also have special ground gauge pins to assist checking your result (separate product). For the above example a gauge pin of the size 0.2405″ would used to check the result and if some cases are found still too tight and has therefore happened because of more springback, then those cases would need to be expanded again using the next size fatter expander to achieve the result you need. Just for those few items of brass.
  • This is why owning a couple of these expanders may be needed to establish a result which you want. Remember actual brass springback is a difficult thing to know beforehand. Also if you are annealing brass properly there definitely will be less springback than regular.
  • These expanders screw into the KM Press adapter (sold separately) for general reloading presses. For those long cartridges (eg 30-06 length) the addition of the KM Window Riser (sold separately) is also worthwhile.
Sizes chart: PLEASE NOTE, the KM part number and list in the dropdown menu refers to the RESULT of the bullet tension after your expanding.
It does NOT refer to the actual measured size of the steel expander itself. But I am listing the actual measured sizes which I have taken, so for those people who want to know this you can correlative a choice based on this knowledge. The “BT” listed doesn’t guarantee this is the result which you will get because springback of different types of brass varies, often significantly.
 -0.0015 BT = result approximately. Actual size of this expander measures approx 0.2425″
 -0.0020 BT = result approximately. Actual size of this expander measures approx 0.2420″
 -0.0025 BT = result approximately. Actual size of this expander measures approx 0.2415″
 -0.0030 BT = result approximately. Actual size of this expander measures approx 0.2410″

These tool are especially helpful for those reloaders who choose not to outside neck turn their brass to uniform wall thickness. If you are also using the reloading dies with the interchangeable neck size bushing those dies usually do not have an internal expander system at all. So you will probably need to use something like this to gain much more consistent bullet grip. (neck tension). Consistant bullet grip plays a critical role in consistent accuracy and velocity. If you are using with reloading die having the interchangeable neck bushings, a smaller size bush than regular will work best for you when combined with using these expanders. In other words resize slightly more so that these expanders can…expand the neck out just a little. How much more? Maybe a bush size down 2 numbers down. Resizing bushes are usually 0.002′-0.003″ under the measured neck with bullet seated. So for this operation i think you will find that should now be 0.004′-0.005″ under. A bit less for the calibres of 6mm and smaller.

  •  There are no Exchanges or Refunds on this item as manufacturer deems these non exchangeable.

NB: We are disappointed to have to say this, but if you will be using Hornady brass in either 6mm or 6.5 Creedmoor with any good dies (in fact especially any Neck bushing type FL die) and then expect that you can use an expander system to help overcome the irregular effects of this brass, you will be disappointed and frustrated for sure. We feel forced to mention this because we have had to deal with some many reloading problems which some customers are having. After buying lots of nice dies and other nice tools like this and get poor results. It’s all because of poor brass. The brass might look good externally but it has very high springback and is particularly tight in the shoulder/neck junction area. No expander will correct it. It springs back to where it was and the reloaders don’t figure out this is what happens. Bullet seating will be irregular and inconsistent and some bullets (particularly Hornady ELD’s etc) and will show compression rings where the seater stem contacts if bullets are seated below this area. If you wish to do a some precision reloading and precision shooting…BUY GOOD BRASS is our advice from lots of experience

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