Neck and shoulderz

[10mm Witness]

I have an idea for a wild cat cartridge that I would like to try. I can have a chamber reamer made for dimensions of my idea. I would use any of the large SA magnum cases based off the .404 Jeffries casing and neck the case up or just use the .404 Jeffries case and neck down.
I have 2 questions that come up.
1. most rifle casings have a 35 degree shoulder. By just changing the caliber size the length from base to the beginning of the shoulder can change or the shoulder anhle can change. Which is more important to stay the same while the other dimension changes?
2. When choosing the case to use, is it easier to neck down or neck up a cartridge?

P.S. I dont like telling people my ideas as I will hear the, 'Dude! You cant do that!' So I just do it. If it works it works and if it don't then I can always sell the bad idea for scrap.

[TurnipEaterDown]

Generally factory brass is short (supposed to be Below min SAAMI Chamber spec), so if you don't want to fiddle w/ headspace correction, and you have plenty of capacity for what you want, on a beltless-rimless case draw your reamer print to put the intersection of the new shoulder & neck BACK (toward base) about 0.010" from the geometrical intersection of the new neck on the Parent case Chamber Print. This way you can size at slight crush fit right from the get go w/ new brass.

Neck down or up, depends on how far you are going. Most recommendations suggest necking down, but both work.

When necking down, look for brass thickening on the neck, and Same Goes if you push back a shoulder or Open a Neck to the point where you use a former shoulder section as part of the new neck. Getting a thick neck from Die Forming is somewhat good if you don't mind more work: turn the neck for best fit & concentricity before firing.

Do NOT trust dimensions in reloading manual for dimensions on brass/chambers. Go to SAAMI website (just search it) and get the proper parent case drawing to work from.
I did a simple case form recently to get brass for what is normally a seasonal run case, and the first few went great. Last one pushed forward because the shoulder was no where near where the misprinted manual placed it, jammed the bullet into lands, fired way over pressure, and sent me on a trip to the gun smith. Reminded me to look at Record prints from SAAMI.

[10mm Witness]

Yes, I am using SAAMI specs. Only way to do this correctly.�� it all makes sense now. Thank you for the reply

[stubshaft]

Easier to neck UP. You only have to fireform as long as the shoulder datum is the same.

[cwtebay]

I prefer necking up, one can utilize fire forming. Looking at Roy Weatherby's work, as well as evolution of the wsm and rsaum cartridge families would likely be a great place to look into advantages of a given shoulder profile for your intended project.

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[pworley1]

Most of the Ack improved wildcats just involved changing the shoulder angle and tacking most of the taper out of the case.

[country gent]

When you look at the more modern and recently designed cases the neck forms a venturi effect where the first powder gas hits inside the neck, This is thought to lower throat erosion and wear. Shoulder angle can also affect the efficiency of a cartridge as in the ackleys the ppcs and the new short magnums. Ackley set his up with the same head space datum for several reasons, the original factory round could still be used, This also saved on special forming dies since factory ammo can be fire formed.Cases werent stressed as much. There werent as many modifications to the rifle feed an magazines this way.

When reforming there is a set amount of brass in a given case, going smaller it can move in and up so it gets thicker and longer. Going bigger it can move out and since the same amount of brass is filling more area it gets shorter. This is apparent with the ackleys removing the body taper takes more brass and the cases shorten some.

[357Mag]

10mm -

Howdy !

First off, and not to sound critical..... " most " rifle cases don't necessarily feature 35* shoulder angles. There are a variety of shoudler angle that have been / are being used. Examples: .223 has a 23* shoulder angle. The .22-250 has a 28* shoulder.
As a wildcatter, you would pick the shoulder angle you want your case design to feature.\

The base-to-shoulder dimension is another case feature you would pick. If your " parent " case has a certain shoulder angle, it would be your choice whether you kept that same shoulder angle on your wildcat; or chose a different one. The " Ackley Improved "
series of wildcat case designs feature what is called " sharper shoulder angle " than what their parent cases had.
Example: the stock .22-250 has a 28* shoulder angle, while the " .22-250 AI " features a 40* shoulder. The designer in some cases would need to pay attention as to whether his wildcat design has an adequate neck length for the chosen calibre / bullets.
That is one example of where the base-to-shoulder dimension might need to be shortened to provide adequate neck length for a given shoulder angle. Just changing the base-to-shoulder dimension in and of itself does not change shoulder angle.
Shoulder angle, base-to-shoulder dimension, neck lg and so forth; are all important features of a bottleneck case design.
That's a great thing about wildcatting..... you make the case design choices. The case' specs are inter-related and inter-dependent.

As regards neck up /neck down: as a generality..... when you " neck down " ( especially through multiple calibres), the neck wall thickness [ of bottleneck rifle cases ) will typically thicken. This reality leads to the need to do things like " inside neck ream "
and / or " outside neck turn "...... to arrive @ the final calibre and neck wall thickness desired. When expanding case necks or straight wall case mouths, the tendency is for the brass to thin some; as expansion is carried out through progressively large calibres


Hope that helps ?


With regards,
357Mag