How does shoulder angle/neck length affect a cartridge?

I've read Ackley, a long time ago.
A bolt action rifle was fired recently at a nearby range with the wrong bolt in it. It was a 6.5-284 and there was a .125 space BEHIND the lugs at time of ignition. The brass may have stayed snug against the chamber walls but the bolt moved rearward to fill the gap, very rapidly. No permanent injuries but very loud report and lots of gas and brass came down the raceway and out of the rifle.
Anybody want to try Ackley's experiment with a modern cartridge?
 
PO Ackley fully understood that modern high pressure cases would yield under pressure but he DID fire the thirty-thirty held shut only with his finger on the lever (Yes Jackie, he removed the sliding lug). He even had a good idea where and at what pressure various cases and case designs yielded. Harold Vaughn further quantified the numbers in "Rifle Accuracy Facts" but didn't take the time to test extensively.

Many folks can attest to primers "popping" above the surface of the casehead, very common in the .308 and .284 cases, this "popping" up indicates that the case did NOT yield and that it DID hold forward.


My memory suggests that on loads developing under 45,000psi with modern brass the case will not typically yield. Of course this number is a SWAG at best and even if it's true it also stands to reason that there is a parameter or spread from say 40,000 to 50,000+ where this is true.


In my work with the .243AI and Lapua brass I remember guessing this that number some times ran clear up to 55,000psi based on chrograph data. Some fairly stout loads didn't budge the brass.

With this in mind it must be noted that using factory or new brass, even high pressure loads transmit only part of their load to the boltface.


Of course this doesn't take into account the phenomena of "sudden" or catastrophic shear failure of he brass, the effect of holding until stretching occurs at which point the brass yields nearly completely transferring more load than if it just sprung instead of yielded.


al
 
Tony, that's what I'm wondering; if you put a 22BR case in a 6BR-DX would the resulting case be a 6BR-DX but retaining the full length BR neck?
I have no experience fireforming so I don't know if it would work that way, overwork the brass, or just split the neck.
Click to expand...

There's an article on forming the improved 6BRs. Two or three different methods. I can see no advantage to necking down to 22BR in the process.

If a standard 6BR-DX reamer is used to chamber the barrel, the resulting improved case would end up the same. Not being a gunsmith, the only way I know of to get a longer neck would be to have a reamer with the longer neck dimensions- which just about gets one back to the basic 6 BR.
 
I was guessing that if the neck was undersize and blown forward with cornmeal that it would stretch forward into the freebore of the throat as it was expanding. There has got to be a way to stretch the neck of the 6BR-DX forward to get it back to the original length of the 6BR. I realize that I would be having a new custom reamer cut but odds are I would have to do that regardless.
 
Ginner said:
I was guessing that if the neck was undersize and blown forward with cornmeal that it would stretch forward into the freebore of the throat as it was expanding. There has got to be a way to stretch the neck of the 6BR-DX forward to get it back to the original length of the 6BR. I realize that I would be having a new custom reamer cut but odds are I would have to do that regardless.
Click to expand...

You could make the neck longer (and thinner) by "ironing" it. Put it on a mandrel and rotate while applying pressure to opposite sides with hardened rollers. I think that is how .45-70 brass is made longer to become .45-90 brass.
An easier way has been developed by someone that no longer posts on these boards. He simply ran a 6BR reamer in deep enough to accept a BRX headspace gage. He made brass from Tubbs 6XC brass, ended up with a long neck BRX. I've heard that others are now using 6X47L brass for this chambering.
 
Ginner said:
There has got to be a way to stretch the neck of the 6BR-DX forward to get it back to the original length of the 6BR.
Click to expand...

IMO what you suggest can not be done by fireforming. You stated you were a rookie so I assume you have little experience wildcating rifle cases. You will discover that necks grow very little in length with sharp (40 deg) shouldered cases. When you blow out a case to less body taper and a sharper neck the OAL of the case will normally shrink, not grow.
 
You can do it quick using a Lee-Six forming die set.
I piddled around with it and made a fat 6br with 284 brass easily. For your project the same could be done from 243 brass and then cut/trim for desired neck length..
Or you could form from 6br brass, giving up capacity for neck length.
You could use a false shoulder for fire-forming with an extra expansion/neck sizing step. Final shoulder angle is set in your reamer.

You can review claims of gain in cartridge ratios by looking up US Patent 6,595,138 -WSM ratios
Flip on your BS filter switch before reading though, cuz there is alot of embellishment here that in my view should have disqualified the claims for serious consideration.

There are also notions that desired flamepoint should be included in cartridge design. Like the successful 6PPC at 73.7% up the neck..
Fp =((Cal/2)/(TAN(ShldrDeg*PI()/180)))/NkLength
So
For 6PPC Fp, required neck length =((Cal/2)/(TAN(ShldrDeg*PI()/180)))/0.737
or required shoulder angle =ATAN((Cal/2)/(NkLen*0.737))*180/PI()

All I know is these considerations don't hurt anything, so I used them with my 26wssm
 
My simple rules of thumb:

I use many 40 deg shouldered wildcats and they all show one similar trait. Very little case growth ie rarely ever need to be trimmed. Most are shot at elevated pressures.

All my standard cartridges like the 223, 308, 3006 need case trimming every 4 to 6 firings. I almost always use a Lee collet neck die so there is no growth caused by the pulling the expander ball through the neck. I feel the shallower angle allows the brass to flow forward easier.

Depending on the action, the shallow shoulder angle and case taper make for smoother feeding. Controlled feed action can be quite fussy about AI chambers. Push feed, especially with center feed mags, are less fussy.

The steep body taper makes for easier extraction especially in auto firearms. Because of this, I would say that a case with more body taper should have more bolt back thrust. It wants to squirt back from the 'thrust' of the burning gunpowder.

As to wear and tear, nothing conclusive as I rarely own a pipe that long. Although more powder burnt for the cal size always wears faster.

I prefer sharp shoulders for single feed guns. Shallower for those that need to run out of the mag.

Jerry
 
Jerry, please explain what you mean by brass "flowing forward" on the shallower shoulder angles.


what's the mechanism, what causes this "flow"? Your use of "flow" seems confusing, like the brass is squished forward when you fire........


al
 
Some Thoughts.

As many of you know, when you turn the necks on a 220 Russian, then fireform it in a typical 6PPC Chamber, the OAL will usually be about 1.490.
When I fire the cases with a 22 bullet in my chamber first, before turning, they all come out about 1.500 to 1.505.
Our first experience with the 30 BR involved Randy Robinettes reamer. It has a shorter trim to length than the originol 1.560 of the 6BR case. This is because when you expand the necks with a mandrel and press, the metal moves back. This is how we first made our cases.
Then We built a fireforming barrel to blow the cases out to 30 caliber, and guess what. They all come out close to the originol 1.560 length.
If I would have known this, I would have gotten Dave Kiff to make the chamber that long, so we do not have to trim so much off of the OAL. Blowing them out first seems to make a better case.........jackie
 
Can anyone tell me where to look at a diagram for the 6brx-dx or dz?
 
mysticplayer said:
My simple rules of thumb:

I use many 40 deg shouldered wildcats and they all show one similar trait. Very little case growth ie rarely ever need to be trimmed.

All my standard cartridges like the 223, 308, 3006 need case trimming every 4 to 6 firings. I feel the shallower angle allows the brass to flow forward easier.

The steep body taper makes for easier extraction especially in auto firearms. Because of this, I would say that a case with more body taper should have more bolt back thrust.
Jerry
Click to expand...

Mr. Mysticplayer and I agree on all the above.

Con
 
Ok Con,

Would you please explain what you mean by "flowing forward under pressure"........ keep in mind that this is an old and well understood subject :)

Also keep in mind that many of us fire shoulder angles as low as 30degrees with no lengthening of the cases.

al
 
I agree with Jackie on making cases, and I would add that if you have a .262 neck, turning at .22 caliber ,before loading to fireform, so that there is a little left to turn at 6mm, helps keep things even straighter, beccause it reduces the amount of expansion needed to do the final turning. I have done it both ways. Average case crookedness is directly related to the amount of mandrel expanding that is done. The main effect of this crookedness is an uneven cut on the shoulder when necks are turned.
 
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