Stupid Question Saturday..... Bushing Neck sizing and Mandrel. Which one first and why?

[ReaperDriver]

Ok, stupid question because I haven't really grasped the mandrel die concept yet.....I just picked up a Wilson Neck sizing mandrel die and a 6.5mm and a 6mm (for a future 6CM build that is in progress). From reading here as well as loading mentor saying the same - the order of the neck sizing is: Neck bushing without the expander stem and then the neck mandrel last.

I'm trying to wrap my head around that because it seems counter intuitive. I get that the mandrel gets the inside of the neck perfectly round and reduces runout. But why would you do that last??? I typically think of the neck busing as a way to control neck tension as some brass needs more or less sizing depending on the case wall thickness. But if your mandrel size is fixed - like in my case with the Wilson, you order a 6.5mm mandrel stem (which measures .262). So if I understand it correctly - this process always gets you .002 neck tension and have no control if you wanted more or less than .002. Is this correct, or am I missing something fundamental?

 

[XLR308]

Size then run the mandrel through the neck.
It nets you consistent neck tention if you dont neck turn your brass.
Expander mandrel gives .001" and turning mandrel gives .002" neck tension with my 21st century titanium nitride mandrels.

 

[918v]

You do that last because the bushing forces all the imperfections to the inside of the neck and because the bushing does not produce a straight neck wall.

 

[OREGUN]

If you have a mandrel neck sizing die, you don’t need the bushing die. The neck sizer squeezes your neck down around a mandrel to create “neck tension” without sizing the rest of the case.

If you use the bushing die to size the case and squeeze the neck, then you would use an expander mandrel in an expander mandrel die body to smooth the inside of the neck and bring it back out to the “neck tension” that you want.

 

[ReaperDriver]

If you have a mandrel neck sizing die, you don’t need the bushing die. The neck sizer squeezes your neck down around a mandrel to create “neck tension” without sizing the rest of the case.

If you use the bushing die to size the case and squeeze the neck, then you would use an expander mandrel in an expander mandrel die body to smooth the inside of the neck and bring it back out to the “neck tension” that you want.

I have an expander mandrel die.

Expanding Mandrel Die-EXP-

 

[918v]

Congratulations

 

[918v]

If you want more tension, get a smaller mandrel. If you want more bullet grip without changing the mandrel size, a tighter bushing will produce more spring back in the neck and more grip on the bullet.

 

[ReaperDriver]

If you want more tension, get a smaller mandrel. If you want more bullet grip without changing the mandrel size, a tighter bushing will produce more spring back in the neck and more grip on the bullet.

Ah, that makes a bit more sense. I hadn't taken spring back into account. I figured even if you used a smaller and smaller bushing, the expander mandrel would always set it to whatever it's diameter is and without different mandrel sizes, you were stuck with that fixed amount.

 

[spife7980]

You size down the large fired neck with the bushing first because if you didn’t then the mandrel would just float freely in free air.

 

[918v]

Ah, that makes a bit more sense. I hadn't taken spring back into account. I figured even if you used a smaller and smaller bushing, the expander mandrel would always set it to whatever it's diameter is and without different mandrel sizes, you were stuck with that fixed amount.

It would do that if the brass was dead soft. But it isn’t unless you over anneal it.

 

[OREGUN]

I have an expander mandrel die.

Expanding Mandrel Die-EXP-

Excellent. I was going off your original post which said “I just picked up a Wilson Neck sizing mandrel die”. A neck sizing die is an entirely different animal. Sounds like you are on the right track.

 

[OREGUN]

A tighter bushing doesn’t produce more spring back…spring back is a property of the brass without regard to how small a bushing you jam it into….until you work harden it or anneal it.

 

[918v]

A tighter bushing doesn’t produce more spring back…spring back is a property of the brass without regard to how small a bushing you jam it into….until you work harden it or anneal it.

Wrong.

a tighter bushing will cause the neck to spring back more and increase seating pressure.

 

[OREGUN]

Wrong.

a tighter bushing will cause the neck to spring back more and increase seating pressure.

So your assertion is that spring back is a function of the “size” of the displacement from some original size/shape?

 

[OREGUN]

My contention is that brass will move about .0005” to .001” opposite the direction you just pushed it with your force (bushing, mandrel, burning powder) once you remove that force. You can use whatever tiny bushing you like but once you run a .264” mandrel in there, your inside diameter will end up around .2635”.

 

[918v]

So your assertion is that spring back is a function of the “size” of the displacement from some original size/shape?

To a point.

For example, if you size the neck to say .284” OD and then expand it with a .262” mandrel the seating pressure will be much higher than if you sized the OD to .290” and then expand it with a .262” mandrel.

 

[XLR308]

My Lapua 6.5CM brass that has been neck turned and run through a .289" bushing measures ..289" after three firings without annealing.
My chamber that was cut by Kelblys i believe is min spec since it is only .002" over headspace on new lapua brass and fired brass is worked very little after firing.
A slighty sloppy chamber would result in much different results.