Mandrel Size

[snowcreekkennels]

I know how to get size of bushing for outside of neck. I don't know what the formula is for finding mandrel for inside diameter. Any formula help for figuring the mandrel size would be greatly appreciated. I'm sure that I am over thinking.

 

[PddPdd]

Most choose to size the neck ID between .001-.004" under the caliber diameter. For example if you are loading 7mm bullets: .284-.002" = .282", so use a .282 mandrel for sizing.
The other thing to understand is that no matter how small you size the neck ID, when you seat the bullet it acts like a mandrel as well to increase the neck size enough (permanent deformation) to limit the actual neck tension/bullet grip/hoop stress in the neck material.

PM me if I just raised more questions and didn't provide a useful answer.

 

[Slim13]

Most choose to size the neck ID between .001-.004" under the caliber diameter. For example if you are loading 7mm bullets: .284-.002" = .282", so use a .282 mandrel for sizing.
The other thing to understand is that no matter how small you size the neck ID, when you seat the bullet it acts like a mandrel as well to increase the neck size enough (permanent deformation) to limit the actual neck tension/bullet grip/hoop stress in the neck material.

PM me if I just raised more questions and didn't provide a useful answer.

I think you explained it great!

As someone who doesn't neck turn, the final operation in my case sizing routine is to run a mandrel .001 under whatever caliber I'm working with. It pushes any neck imperfections to the outside of the case wall and produced very concentric ammunition. I can see the difference on the target but ymmv.

 

[just_jon]

selecting die bushings - Search Videos

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[Mikecr]

1thou under cal has always worked well for me.
I would still prefer a hardened mandrel right at cal.

There is no formula for any of this. Just anecdotal setting of interference, given normal spring backs.
Normal spring back(either way) is ~1/2thou.
So for example, let's say a conservative reloader does not want to use his bullets for up-sizing of necks. And he wants high consistency reasonable neck tension, so he's not going to be annealing constantly, but minimally working his necks. He also will never size length of neck beyond seated bullet bearing.
In 22cal he measures loaded neck diameters of .248 with 12thou neck thickness. If his neck downsizing to desired, from fired dimension, is 5thou or less, he can go with a bushing stamped .246. The necks will spring back to ~.2465 OD which is ~1.5thou under cal interference.
If he were then to run a mandrel at cal (.224) through sized necks, they would up-size & spring back to ~.2475, leaving 1/2thou of interference.
With that, his seating bullets would expand necks but would not be up-sizing at all. A perfect pre-seating.

I know this would work fine, but you can add any effort you like from there. Maybe a .2455 bushing/.223 mandrel.
From there keep in mind that a circle of excess sizing (up/down, and lengths) and annealing, feeds on itself in the long run. You can be sure of that much.
If bushing sizing to desired has you downsizing >5thou, go with a slightly higher stamped bushing. This much sizing angle rolls the necks inward, past that stamped on the bushing. That's common but it works necks a lot.

 

[HuntingBronco]

Do the answers here depend on bullet design/make? Seems like Hammers like more tension - do Bergers like less?

 

[Mikecr]

Tension affects the load/peak pressure.
I don't see how it would make any difference to a bullet.

As far as seating force (which is not tension), I'm sure jacketed bullets appreciate less.
We're not supposed to be using our bullets as neck expanders.

 

[RUFFIAN R - NY]

a few factors to consider are you reforming brass 06 to 25 cal or to 6mm - brass will thicken up. Do I want to do inside or cut outside - what's my chamber / throat ? Brass itself will make a difference between manufacturers and in lot numbers also !

 

[Ray Gross]

The force required to release the bullet from the neck is a tunable attribute just like seating depth, jump, charge weight, powder choice, primer choice, etc.

Expanding the neck to a lesser of greater degree with a mandrel, will tweak that force. Depending on degree of change and the other attributes, expanding the neck to s certain diameter may or may not have any material effect on precision and consistency.

 

[Mikecr]

In my opinion only, it's better to minimize up/down sizing for necks, and adjust tension through sizing length.
What grips our bullets is springback force against an area of seated bullet bearing. Instead of trying to change the force only, we can apply force to more or less area. This seems easier to me than trying to adjust the only attributes of spring force (with precision). Especially given that we have no way to measure the force.

 

[QuietTexan]

Do the answers here depend on bullet design/make? Seems like Hammers like more tension - do Bergers like less?

Change out tension for engraving pressure. We use tension to describe a general fit of the neck around the bullet, but the quantitative internal ballistic measure being adjusted is peak chamber pressure by controlling the timing of the bullet engraving into the lands of the rifling.

The force required to move a bullet down a barrel is enormous, for a copper bullet it can be over 1000# of push force to move the bullet through. There was an old report from the military I read that was a test of different rifling designs, it included this nugget:

The increase in peak chamber pressure could be reduced by either increasing the free run of the projectile prior to the start of engraving or by reducing the forcing cone angle in the barrel.

So when you say Hammers like more tension, what's really going on is that in a tighter neck and when sat deeper, the Hammer has a lower peak chamber pressure because of the increased free run to engraving from both the deeper seating and the longer time frame before the bullet is released from the case. This is logical because the Absolute Hammer was designed to have the first engraving point on the first PDR, not anywhere on the meplat of the bullet.

The forcing cone concept is the root of Shawn Carlock's +P rifle chambers with a modified leade section of the barrel.

 

[HuntingBronco]

Thanks QuietTexan. That also helps why Crimping has, in some instances, shown to be beneficial. All about varying the push to achieve your goals.

 

[RYEWSKY25284]

I know how to get size of bushing for outside of neck. I don't know what the formula is for finding mandrel for inside diameter. Any formula help for figuring the mandrel size would be greatly appreciated. I'm sure that I am over thinking.

I go .003 allowing for .001 spring back netting me.002

 

[Ray Gross]

I know how to get size of bushing for outside of neck. I don't know what the formula is for finding mandrel for inside diameter. Any formula help for figuring the mandrel size would be greatly appreciated. I'm sure that I am over thinking.

It depends on the ductility of your brass necks. Lately I have been loading a lot of 308 Lapua brass and I use a mandrel that measures .3065, which give me a neck ID of ~.3060.

The mandrels are like neck bushings, you end up with 2 or 3 extra for every one you actually use.