Neck sizing theorycraft

[mattj]

I've been thinking about neck sizing and about a better way to do things -- I'd appreciate if some of you would be willing to read over what I've been working over in my head, and encourage me and/or poke holes in the theory as appropriate...

So, if you don't turn necks, you are going to have a degree of irregularity in neck thickness, the degree of which depends on luck, how anal you are about culling, and the quality of your brass...

If you use bushing neck dies, or have a custom honed sizing die, you'll be sizing the outside of the neck, but bumps and irregularities will be pushed to the inside of the neck, and also the inside of the neck will likely not be of uniform diameter due to the varying neck thickness of your unturned necks -- and in my understanding, this can't be good for consistant neck tension, and possibly runout.

One alternative is the Lee Collett neck sizing die, which squeezes down on a mandrel inside the neck -- since you're shaping based on the mandrel on the inside of the neck, after sizing with this die the inside of the neck should be consistent diameter, and all of the irregularities pushed to the outside. This is all good, but my brain got churning on an alternative.

So lets say you have a full length sizing die with the expander ball removed, but has been honed such that after sizing, the neck has been sized down just enough such that the thinnest neck-thickness brass that you allow (after culling) will have its neck resized after springback such that it would create just a bit tighter neck tension than you actually desire. This ensures that even the thinnest walled brass will be sized to have neck tension slightly higher than that which you actually desire (if the bullet were seated at this point).

So, then you run the above sized brass through something like a Sinclair expander body with a mandrel of appropriate diameter (my guess is that the oversized neck turning mandrel, which has bullet diameter minus .001", should spring back just enough to give you .002" of neck tension) to uniform and appropriately size the inside of the case neck. The slightly tighter neck sizing that occurred in the previous step insures that even the thinnest-walled brass gets sized up to the appropriate inside diameter/seated neck tension in this step.

I've seen lots of references to using the expander body/mandrel BEFORE using something like a neck bushing die to size the neck -- but it seems to me that sizing the inside of the neck last should make for the most uniform neck tension, especially if you don't turn necks, as you are explicitly sizing and uniforming the inside, as opposed to the outside of the neck and hoping the inside turns out okay. This should get you the positives of an expander button, without the negatives (ie, hopefully you're working the brass less due to your honed die, and you avoid the run-out problems associated with expander buttons).

I'm hoping that this will prove a relatively quick way of getting low runout with consistant neck tension by avoiding some of the pitfalls of other methods, and avoiding neck turning (Note: Assume reloading for a no-turn match chamber)

Any gaping holes in my understanding or in the described method? I just wanted to bounce this off you guys before I spend money testing the theory...

Thanks!
-Matt

Edit: Just a note, I realize I'm not just neck sizing here (assuming that I'm using a honed full sizing die in the initial sizing step) -- Intention is to still to full length resize every time for feeding reliability (though with die adjusted to only slightly bump the shoulder back) -- I'm just focusing in on how the neck gets sized here.

 

[Maico]

Matt....sounds like you need to buy a Redding FL "S" die that uses the bushing for controlling the neck size. In my opinion they are the best threaded die on the market.
If you're shooting a factory rifle you'll not notice neck thickness much but if you're shooting a custom rifle then then neck thickness is a concern and most custom rifles need the necks turned. I measure my necks"for factory guns" and say the low side is .012 and the high side is .0135 I'll set up and turn the necks just enough to take the high side off. Most factory chambers are very oversize and it works the brass more. Hope I've helped you out some.

 

[Forester]

I think your method would probably work, but wouldn't the work hardening of your brass affect the spring back on the neck and require either frequent annealing or a collection of different mandrels?

The Lee Collet die works like a charm and is cheap...I may well be missing something but it seems to me that your proposed method does not accomplish anything that the Lee dies doesn't.

I am also a bit averse to any extra steps in the process because every step you add is another opportunity to introduce runout.

 

[mattj]

Thanks for the replies guys.

Forester -- The Lee collett die is definitely a consideration, and I may be silly for trying to re-invent the wheel here. What I've read about it makes it sound a bit 'finicky', which turned me off a bit... but maybe I just need to try the thing before passing judgment -- not like it is un-affordable.

That said, I don't think my proposed method has a lot of extra steps, given you plan on full length sizing every time any way -- Full body (honed) + expander mandrel vs. Body-only die + Lee Collett -- same number of passes through the press.

I do see your point about working the neck. My theory was that by using the honed die to size it down *just enough*, versus what happens with an un-honed die where it gets undersized significantly before being buttoned back up would minimize the amount of re-sizing -- granted I have not yet determined exactly how much less this custom honed die would actually be working the brass. I guess that with a body-only die + Lee Collett you are only resizing the neck once per reload, instead of what I'm talking about which is sizing in then out. Forester, I think you might have just pointed out where I'm off my rocker a bit -- I figured I was, just couldn't see how. 'preciate it.

 

[keithcatfish]

Get yourself a Lee Deluxe Die set (collet die, full length die and bullet seating die) for 25 bucks and have the neck on the full length die ground a few thousandths larger than the diameter of a fired case neck. Viola- instant body die and no extra cost.

 

[Forester]

Thanks for the replies guys.

Forester -- The Lee collett die is definitely a consideration, and I may be silly for trying to re-invent the wheel here. What I've read about it makes it sound a bit 'finicky', which turned me off a bit... but maybe I just need to try the thing before passing judgment -- not like it is un-affordable.

That said, I don't think my proposed method has a lot of extra steps, given you plan on full length sizing every time any way -- Full body (honed) + expander mandrel vs. Body-only die + Lee Collett -- same number of passes through the press.

I do see your point about working the neck. My theory was that by using the honed die to size it down *just enough*, versus what happens with an un-honed die where it gets undersized significantly before being buttoned back up would minimize the amount of re-sizing -- granted I have not yet determined exactly how much less this custom honed die would actually be working the brass. I guess that with a body-only die + Lee Collett you are only resizing the neck once per reload, instead of what I'm talking about which is sizing in then out. Forester, I think you might have just pointed out where I'm off my rocker a bit -- I figured I was, just couldn't see how. 'preciate it.

I missed that you plan on full length sizing every time anyway, so I was thinking about my method which is to use only the collet die until the cases become hard to chamber and then bump the shoulder and go again..I can usually run fairly hot loaded .308win about 4 or 5 loads before I need to bump the shoulder.

I don't currently neck turn, but I have the tools and may do some experimenting soon. For that my plan is to turn the neck turn mandrel down to .001 smaller than the mandrel on the collet die and only really hit the high spots-which there are not many of on the Lapua brass I'm using now.