Question on expander mandrel

[TylerFromMS]

Could uses a little improvement . . .

. . . he says jokingly.

I originally set out to see if a mandrel was necessary, because that’s what I had always heard. It just shot so well without it, that I ended up just messing with the bushings. No neck turning, of course Harrels die.

 

[Webster]

what size expander mandrel 6.5 creedmoor for .001 neck tension?

Tension (grip) cannot be measured with a micrometer. Depends on work hardening and neck thickness.

I have a 6BR used for bench shooting and varmint hunting. It's a no turn neck chamber. I still turn the necks to 0.0125". That's only 1-1.5 thou removed. You can take a skim cut (just cleaning up about 75% of the surface) to just even up the thickness. Never sized with a mandrel or ball except a mandrel to prep for turning Always get groups no bigger than 0.400” with good bullets. No guarantee turning or mandrels will produce better groups. The K&M neck turning tool comes with an expander mandrel to try to get a tight fit on the neck turning mandrel. I use a Redding type-S bushing die. Also bought about 5 steel bushings of various diameters. I don’t think carbide dies are necessary. I lube the necks. Current price for the turner is $115 at Midway. I didn’t pay anywhere near that price 15-20 years ago. I anneal about every 3 firings, neck splitting not a problem. I would definitely buy the Redding type -S die. Try the rifle without neck turning and see if you can work up loads that are acceptable. If I was starting a new rifle, I would buy Berger bullets for a first try. The Sierra 60 gr. Varminter shoots as good as match grade bullets in my rifle.

 

[Ned Ludd]

In my hands, the use of a mandrel as the last neck sizing step buys a little better consistency in neck tension, and the necks end up nice and straight. That's about it. You can feel the difference when seating bullets over time between necks that have only been sized with a bushing die, and those that have been sized down first with a bushing die, followed by the mandrel. The bushing + mandrel approach gives me fewer outliers based on seating force. The difference isn't huge, but it isn't zero, either. Quantifying a statistically significant improvement in scores using the bushing + mandrel approach may not be an easy feat.

As David Christian noted, both the bushing and the mandrel actually need to do some work and move the brass. For that reason, when adding a mandrel sizing step, you'll probably want to start with a bushing that is at least .001" under the size you would use if the bushing die were to be the sole sizing step. Because spring-back of the brass after opening necks up with a mandrel works in the opposite direction as compared to the use of a bushing die, the necks will close up slightly after the mandrel is withdrawn. Thus, using a mandrel that is ~.0015" under bullet diameter should generate pretty close to .002" neck tension (interference fit). Having two or three different bushings in .001" smaller increments from the size that would be used if the bushing die were the sole sizing step, and two or three different mandrels that are something like .001", .0015", and .002" under bullet diameter should be enough to experiment and determine the optimal combination of bushing and mandrel for a given cartridge.

 

[josebd]

That's really amazing. What kind of cases do you use? Do you measure your necks?

Lapua