Help getting set up to use mandrels

[DartonJager]

Would like to give using mandrels to expand my case necks a try and know little about it and would like to know what I need to buy.
Have decided to go with 21st century Mandrels and die body but now for what I don't know.

I wish to start with using mandrels for my 300wsm and need to know how do I determine what size mandrels to buy.
Secondly will my Redding FL resizing die with the expander removed work to reduce the neck ID and be compatible with using a mandrel to enlarge it so as to give me between 0.0015 and 0.002" of neck tension.

Lastly is setting my neck tension between 0.0015 and 0.002" my best choice for best results.
I am researching this quite a bit and so far all info and videos I have found only show how to use mandrels not how you determine what size to buy.

 

[Jud96]

I use the Sinclair mandrels and mandrel die but they work the same as the 21st century setup. This is how I determined what mandrels to buy. Take one of the pieces of brass you plan to use, seat a bullet in it, then measure the outside diameter of the loaded neck. Let's say for example it measures 0.336". If you want .002 neck tension I would order a 0.3345" mandrel. I found that there's a little bit of spring back when sizing with the mandrels. My .002 under mandrels actually give me about .0025 of neck tension, which works perfect for me. I also size my brass with standard dies with the expander ball assembly removed from them. I don't use a bushing die. Some say this overworks the brass but it's been working flawless for me. Finally, to answer your question about neck tension. Personally I have always ran .002-.0025 neck tension and have never had any issues. Most people run between .001-.003 of neck tension. I just stick in the middle of that and haven't had a problem. You could always experiment with it, but I believe as long as you're in that .001-.003 range and you work up a load using the same consistent neck tension, then you'll be golden.

 

[Mikecr]

The best neck expansion is really a Pre-Seating operation.
That is, using an expander mandrel for neck up-sizing instead of bullets (which are not made for this).
While most people do up-size necks with seating of bullets, it's not ideal.

What grips bullets is no more than neck spring back force against an area of seated bullet bearing. Spring back is ~1/2thou. And when you seat a bullet in a neck, regardless of the interference size and force to do so, then pull that bullet, the neck will spring back that ~1/2thou from cal.
So to size minimally, while keeping full tension, you would run an expander mandrel through necks at the same diameter as your bullets (at cal). You can get .308 mandrels from 21st: http://www.xxicsi.com/caliber-specific-expander-mandrels.html
After expansion, the necks will spring back ~1/2thou, ready for actual bullet seating.

 

[DartonJager]

Superb information Jud and Mike, thank you a great deal. I am not only able to order what I need I also now have a decent beginning of understand how and the why of mandrels.