Optimal Shoulder Angle

22 March 2003, 08:39

The shoulder angle of bottleneck cases certainly shows one of the greatest ranges of variation among various cartridge designs -- from very shallow angles like on the 30-06 to 40 degree Ackley shoulders or steeper. It seems that lately a lot of factory and wildcat cartridges use 30 or 35 degree shoulder angles, but what do you consider to be the best shoulder angle -- and why.

[ 03-21-2003, 23:43: Message edited by: InfoSponge ]

22 March 2003, 08:54

Paul H

It depends on the cartridge. In general, I don't like shoulders sharper then 30 deg, the sharp sholders don't lend themselves to being moved back well on case forming.

On rounds that have a relatively small bore to the dia of the case, I think a more gentle shoulder is better as it eases feeding from a magazine.

On rounds that have a bore near case dia, a sharper shoulder is conductive to the shoulder not being bumped back during the striker hitting the primer.

To a large degree I think the angle is irrelevant to cartridge performance if kept in the 15-30 degree range.

22 March 2003, 15:20

Savage99

There is a recent article in Precision Shooting that has strong opinions on shoulder angles. This is my report from memory. The author concludes that once the primer lights some of the powder the rest of the powder is forced against the shoulder area and neck of the cartridge.

If the shoulder angle is slight (less than 20 degrees) the critical energy from the primer is not directed into this slug of unlit powder and thus ignition is not perfect. Likewise if the shoulder angle is too steep the same loss of directed primer energy is lost.

The author concludes that the best shoulder shape is a round one and he has applied for patents and is having tests done by Norma.

22 March 2003, 15:38

<PaulS>

Infosponge,
I can't tell you what is best or optimal but I can show advantages and disadvantages for some of the extremes.
Low angles, say less than 15 degrees don't really center the cases well unless they are formed in the chamber at close to maximum pressures for the weapon. This can be shown by using the rise/length ratio or TANGENT of the angle. A 15 degree angle moves forward .010 inches for each .00026 inch of diameter. If you have .002 inches of diametrical clearance then the case can move .076 inches before it will come into contact with the chamber.(with the mean length between the base and shoulder the same) A 45 degree angle is 1 unit forward to 1 unit rise - so the same cartridge with a 45 degree angle would only move .002 forward if it had the same .002 inch diametrical clearance.
On the other side is the internal pressure and how it works on the case. With steep angles (say 45 degrees) the internal pressure begins to treat the case shoulder as an obstacle and moves it forward, stretching the body of the brass to take up the clearance of the case in the chamber. I believe (it is only a belief) the the optimum balance between the two extremes is about 30 -35 degrees because it still centers the brass well while allowing the case to expand into the chamber more or less equally. That allows the brass to return to its shape more uniformly and promotes good case life with the accuracy that we have all come to expect.

PaulS

22 March 2003, 16:13

Dave Jenkins

S-99, Round as in a double or single radius?

22 March 2003, 17:00

Savage99

Single radius measured from the inside of the case. ie the shoulder bulges outward so the energy vectors that hit it's inside are all directed to the slug of powder.

23 March 2003, 05:46

dan belisle

Ah, rather like a hemispherical chamber in a cylinder head? - Dan

23 March 2003, 06:30

Savage99

While I want to discuss engines I am not that familar with the "Hemi" except for it's reputation.

In general the old saying "If you don't know how to make something then make it round, if you can't make it round then make it square" seems to apply.

The author also endorsed short cartridges.

23 March 2003, 20:19

HenryC470

quote:
Originally posted by Savage99:
The author concludes that the best shoulder shape is a round one and he has applied for patents and is having tests done by Norma.

He might run into a prior art snag, viz. Weatherby cases.

H. C.

quote:
Originally posted by Dave Jenkins:
S-99, Round as in a double or single radius?

quote:
Originally posted by Savage99:
Single radius measured from the inside of the case. ie the shoulder bulges outward so the energy vectors that hit it's inside are all directed to the slug of powder.

Maybe not.

[ 03-23-2003, 11:23: Message edited by: HenryC470 ]

24 March 2003, 06:16

Savage99

Henry,

You should read the article.

I wish Ken Howell would comment on the article also.

The Weatherby design has two radius's that, to me, seem to be the opposite effect of what the "inventor" is testing.

In general everything can be improved. Perhaps there is something to this.

24 March 2003, 07:24

HenryC470

Savage,

To tell you the truth, I haven't heard of Precision Shooting. Is it a print magazine or is it online?

H. C.

24 March 2003, 07:40

dan belisle

Print magazine. - Dan

24 March 2003, 07:46

Savage99

They are at www.precisionshooting.com

They also publish "The Accurate Rifle" and many books by esoteric and erudite authors such as Ken Howell. It's pretty deep stuff.

Some of the articles are free as you can read on their site.

24 March 2003, 07:49

Savage99

Click on the Accurate Rifle link and read the article about Askins. That guy was as tough as they come. Never mind Dirty Harry. This guy was the real deal.

24 March 2003, 09:36

HunterJim

I sometimes think that there isn't anything new to do with cartridge cases that has not been tried before. I took a look in Parker Ackley's Handbook for Shooters and Reloaders v.1, and found two cartridges with round shoulders: the 7mm Express by Roy Gradle and the 7mm Venturan by Ralph Payne (Ralph took over from Roy after the latter retired). The Express was formed in an air and hydraulic apparatus, while the Venturan case was formed in a special tool. The Express was made from .348 Win brass (another early WSM!) with the rim turned off, while the Venturan was formed from H&H head size brass. Both cartridge designers stressed the benefit of the round shoulder.

Prior art indeed!

jim dodd

24 March 2003, 12:35

<eldeguello>

A ROUND shoulder?? My, what an innovation. A'la Powell-Miller Venturi-Freebore (PMVF) or perhaps Uncle Roy, who turned the PMVF inside out, and called that an invention?? [Big Grin]

[ 03-24-2003, 03:36: Message edited by: eldeguello ]

24 March 2003, 14:42

Savage99

Yes Hunter Jim I remembered these when the "new" invention was written up.

It's not the Powell Miller Venturi Freebored at all and in fact the opposite. It's a convex not a concave shape.

27 March 2003, 16:05

Nitroman

You can find some very interesting reading in college level physics books regarding gas dynamics.

I think the double venturi shoulder was meant more to keep garage gunsmiths from duplicating W's reamers.

I think a good way of looking at this would be to build a chamber that wold have the cartridge in it and the bullet sticking out. With the bullet gone you could visually see the case mouth. Load up and take Schlerer films of the gas flow for various angles.

That is how I would do it.

28 March 2003, 03:42

Art S.

I read the article mentioned on Askins. It was pretty interesting. However, I don't know what hemisphere this author lives in, but his statement that "Keith respected Whelen and O'Conner" is at the minimum uninformed. He recounts the fact that Keith was often seen with them at Nilo. Keith in his own later work reports that he demanded that Winchester never pair him with O'Conner. He openly claimed that O'Conner almost killed him with a shotgun and generally wrote in many of his letters of his dislike for O'Conner. It almost seemed that he let his dislike of O'Conner shape much of his personal ethic as to firearms.

I grew up as a disciple of O'Conner. My later hunting experience, coupled with the reality of modern hunting, converted me to Keith's view. However, I still believe that O'Conner was the better technician of the two and, due to his position at Outdoor Life, really the more experienced. Keith was a marvelous character and accomplished an amazing amount for the shooting industry, but in his old age became something of an anachronism. His final writing (and correspondence) were very disappointing from a professional and personal standpoint.

28 March 2003, 07:33

Bill Leeper

The convex rounded shoulder would be like that on the Gradle Express cartridges developed by Roy Gradle in the 50s.
In general, variations in shoulder angle are something to be considered if you have too much time on your hands! Regards, Bill.

31 March 2003, 12:41

HondoJohn6508

Here is an interesting link concerning shoulder angles if you are interested in that sort of business that might be well worth reading.

http://www.angelfire.com/ma/ZERMEL/twist2.html

one part is a discussion on the article that Ron Jeter wrote in the May,1998 issue of Precision Shooting Magazine titled, "Turbulence Point Wild Cat Cartridges"
The other part delves into the science/voo doo of barrel twist.

I think the crew that reads this forum would find the article interesting, but then again I could be wrong.

Ol' John

[ 03-31-2003, 03:43: Message edited by: HondoJohn6508 ]

31 March 2003, 14:18

Grandpasez

Big Bores-- best shoulder angle--ZERO..Ed.

31 March 2003, 15:37

RIP

Anything from 15 to 45 degrees, as long as there is enough step down from shoulder to neck to maintain headspace positively.

30 degrees seems to be the all-around best number, but the 45 degree shoulder of the antique .416 Rigby was futuristic in 1911, and sure worked well. It is a slick and smooth feeder with its generous body taper and 45 degree shoulder with generous step. A positive stopper in more ways than one.

In a big bore, the turbulent flow stuff doesn't much matter, and the belted case with 0 degree shoulder, or ghost shoulder, might be the winner if that was a consideration.

My rule of thumb for a bare minimum functional shoulder for a rifle cartridge is the "TWENTY-TWENTY RULE."

20/20: 20 degrees and 20 thousandths inch per side at the shoulder-neck step down slope.

Any shoulder with lesser angle needs a bigger step, but you need to stay at or above 20 thou step (0.020" per side) whatever the angle greater than 20 degrees.

The belt does it with a 90 degree angle and a 0.010" step, a special case, not really a shoulder.

This is from "Redneck Wildcatting 101," and I am self taught.

Sumbuddy who know better rule of thumb?