So, playing around with numbers, I found something rather intersting.
Turns out, if you take bolt thrust as the limiting factor, then a .475 cal of 3.1" COL will be faster with a 404 case than with a Rigby case.
Come up with your own numbers if you like, but I have set the bolt thrust as that of a WSM or RUM at 65k PSI. The Jeffery case is a wee smaller, actually (.555" or .550" vs .545", resp.) so we're actually talking less than what the SAAMI spec'ed WSM would give a M70.
The Jeffery case at 65k gives about the same thrust rearward as the Rigby case at 53k.
IF (big if for some) we accept 65k PSI as comparable to 54.6k CUP and 53k PSI as comparable to 46.8k CUP, then Powley tells me a 24" bbl will give a 500g from the Jeffery case 2450fps, vs 2400 for the Rigby.
Those are pretty high pressures for a DG caliber, yes.
The neck on a 470-404 is marginal at best, yes.
The feeding on a 3.1" Rigby would challenge a master, yes.
But isn't the virtual data interesting?
Doing this all in a .458" would make a bit more sense; .416" even more. And the pressure comparisons should hold true.
If anyone wants to see what QuickLoad has to say about it (without the clumsy PSI-CUP translations), my calculations give the Rigby and Jeffery cases 121.4g and 106.5g water, resp, with a 2.35" case and 3.1" COL, .008/1" taper and 35* shoulder.

Cheers, and Happy Holidays everyone!
T

BB

404 rim diameter - nominal .545
RUM rim diameter - nominal .532

a 2.55" 458 rum case, along the same lines, is 112.1 gr


run your numbers down to 55kpsi max, and then let lug setback be someone else's worry.

jeffe

Rim size isn't the measurement. It's the dia of the inside of the case.
My measurements are estimated extrapolations from a cut 40 case.
Thing is, if you like the Rigby case at 55k, the 404 has less bolt thrust at 65k.... Something to think about. Personally, AEBE, I'll take the lower pressures, and if Winchester chambered the 300 RUM with a 65k SAAMI spec, then I would think - all other difficulties aside - it will handle the Rigby at 53k. And that *migght* get me close to 2400/500 with a 24" tube.
Wish I had QL to see what it says....

quote:

Originally posted by Bwana-be:
Rim size isn't the measurement. It's the dia of the inside of the case.

best of luck.. not the way i've ever heard that part explained before.. from Thompson center to ellis brown.... I called tc abotu 15 years ago to have them make me a 358 wincheter contender.. the answer was a flat NO ... I asked why, and got a lecture on bolt thrust.

have people built them? yep... has TC sold a CONTENDER in one? nope.

Again, best of luck
jeffe

Well, I'm certainly no expert by any means, and woudl entertain any alternate explanations, but the way I hear it is that the ignition exerts force all around the inside of the case, and that which concerns the lugs is the amount of Xpsi that can go back. It won't matter if the rim is rebated or very much oversized, the inside wall is what will allow the surface for pressure backward. If it were the rim, then rebating would reduce backthrust. That doesn't make sense. That being the case, the Jeffery has a slightly smaller casehead and should have a bit less bolt thrust than the RUM which should have less than the WSM at any given pressure.
There are other things, which is why I started this out with, "if you take bolt thrust as the limiting factor." The barrel tenon and front ring, for one, have been brought up as an area of concern with these fat cases. To some extent, seems a good smith/machinist could figure how much to ream out the front ring, and increase the barrel tenon for more support. May not be necessary, with all the high-test M700 Lapua Mags out there (considering this would be a lower-pressure cartridge) but might not add much onto the cost of a custom rebarrel anyway.
The biggest issue I hear about is getting a little 3.1" long .475" caliber cartridge to feed from a standard-width action. My lack of faith in anyone for that job is what usually brings me back to just wanting to build an Mbogo (besides the fact that bigger is better!)

Regarding the Contender, I'm not sure what you're getting at, since not only have their been M70's built on the Rigby case, but by Winchester's custom shop no less. I would think they'd be the first to back away.

quote:

Originally posted by jeffeosso:
BB

404 rim diameter - nominal .545
RUM rim diameter - nominal .532

Jeffe, Are you saying that Rim Dia. has any bareing on bolt thrust???????? roger

When everything is working right, the bolt thrust is not predictable, and will depend on case shape, chamber finish, clearances, lubrication, etc. Ackley improved cartridges may produce little or no bolt thrust.

If you want to design for the worst case scenario, which is what I would do, then you should design for case seperation at peak pressure. In the event of a case seperation, the bolt thrust is going to be peak pressure times the cross sectional area at the point of seperation. Since cases normally seperate at the web, and not at the rim, I recommend using the web OD. Never mind the rim diameter, and never mind the case ID.

In the event of a case seperation, the TC action can and sometimes will pop open and blow hot gas and pieces of brass and primer back in the shooter's face. Ask me how I know. Even the skinny .223 case generates enough thrust to unlock the TC action when a case seperates. The reason people get away with big cartridges in the TC is because cases rarely seperate.

We would refuse to buy a bolt action that blows hot gas and molten brass back in our face when a case seperates, but that's exactly what TCs and some other break open single shots do.

So, how do you "design for case seperation at peak pressure" exactly?
I don't see how it would have anything to do with the web OD. The "peak pressure" at the time of seperation, would still be pressure times inside area (it's the inside dimensions that create the pressure level, and the rear side of that which pushes back) then relieved somewhat into the larger space of the chamber - plus forward bullet movement, etc.
As to not buying guns that spew back at you, well, good advice.
;-)

When the case head separates, peak pressure is applied over not only the ID of the case, but on the edge of the separated casehead. The total area is that of the case OD. In principle, the jet of escaping gases will also push back on the rim as well, so it certainly wouldn't hurt to add that area as well.

If an intact case is well lubricated, the thrust is also the OD area times pressure, for the lubricant film keeps the case from clinging to the walls and also serves to equalize pressure around the case. Unless the rim were very tight fitting, it's diameter wouldn't matter in this scenario.

I for one don't believe that an AI case ever eliminates thrust; the brass just isn't strong enough to retain the head. Well, unless the case OD is very close in diameter to the primer OD, but in that case the swelling of the case around the primer pocket should become a problem instead.

For safety's sake, design thrust for case OD, not ID.

You should design the system to have minimum 3 times safety margin between the thrust against the bolt and the shear rating of the locking lugs, with tensile strength of the reciever greater than the shear rating of the lugs.
example my 12GA FH in the Savage 210 bolt
action gun. Shear rating of lugs about 42,000
lbs thrust, and case thrust at 35,000 psi
peak pressure, is about 11,900 lbs.Ed.

quote:

Originally posted by asdf:
When the case head separates, peak pressure is applied over not only the ID of the case, but on the edge of the separated casehead. The total area is that of the case OD. In principle, the jet of escaping gases will also push back on the rim as well, so it certainly wouldn't hurt to add that area as well.

If an intact case is well lubricated, the thrust is also the OD area times pressure, for the lubricant film keeps the case from clinging to the walls and also serves to equalize pressure around the case. Unless the rim were very tight fitting, it's diameter wouldn't matter in this scenario.

That just isn't so.
The force is applied to the entire rear, yes, but the load that made Xpsi was within the ID, and therefor decreases with the increase in area. If the cas seperates immediately. it will not be able to generate the same peak pressure.

[Ed, thanks for the comment. Know the shear rating of M70 lugs?]

Bwana- I haven't measured lugs on M70, but
they will be about same as Ruger and Enfield, 33-34,000 lbs thrust..Bolts are similiar
with little extra length on left lug.

ASDF-Regular drycases have a coefficient of friction of about .04, and multiplying that times area of the sides in sq inches x peak pressure give the amount of hold the case has to chamber at peak pressure.Even oiled cases
have a coeffient of about .01, and that gives enough bite that is equal or greater in most cases to tensile strength of the case totake part of the thrust..In the 30-30 experiment
Ackley did the bolt was long ways from supporting baseof the case, but yet case held
in chamber, and case didn't stretch back,IE
the strength of the 30-30 cases was more than
the back thrust of the firing due to that
cartridge not being loaded 60k psi, but
only half of that..

I use simple math that is accurate, without
sectioning cases. Base area x peak pressure x
2/3..the 2/3 accounts for the difference in od and id, and the part of the thrust that the case holds when it grips the chamber..Ed.

Bwana-be: if the case ruptures at maximum pressure, that pressure will immediately be applied to the entire diameter of the case. The volume opened by the fractured brass simply isn't enough to relieve the pressure. If the case fails early in the burn, you are correct, the escaping gases may prevent the pressure from reach its typical max.

hubel458: I've read Ackley's book, and frankly I wonder if he wasn't pulling everyone's leg on that one. His explanation of how the AI chamber reduces rearward thrust is bogus. He claims the straight walls reduce the rearward thrust on the brass case. He failed to see that that decrease in wall angle was accompanied by an equal and opposite increase of the angle on the inside of the case.

To do some math on this problem, I'd need to know the cross sections at the point where the bright ring forms, as the case life nears rupture. This is the place where the case is letting go of the walls and yielding. The case forward of the bright ring is that clinging to the chamber walls, and that rearward of the bright ring is that being shoved rearward, unable to get enough purchase on the walls to hold tight. It is the sectional area at this point where the brass must be thick enough to contain the thrust on the case's base. The net area on the case's base is the ID at the bright ring less the diameter of the primer. It may be that given a LR primer in a case of .42" diameter, there is enough brass to contain the pressure, up to a point.

At 60 ksi: I have a hard time swallowing that. The yield point of brass isn't too much above 60 ksi. The difference between the .42" case and the .21" primer is an annular ring .21" thick, and I really don't think the bright ring is occuring in brass anywhere near half that thick. (I don't have a .30-30, so I don't have any brass I can section, sorry.)

Another way: we've got .21" between the primer and the case OD. If the gas pressure is equal to the yield point of the brass (not unusual with hot handloads!), the brass at the yield ring must cover half that sectional area. If I got my math right, that's an ID at the yield point of .33" or a wall about .045", and, well, I, ah, must admit the case at the yield ring could be about that thick, but I remain skeptical.

Alright, for the .30-30, Ackley's claim may not be too far off at 60 ksi. So there!, I may have proved myself wrong. For a belted mag case of .51" OD, I see no way the brass walls are holding back that case head.

Asdf--Your right in that the part of the case
that holds the thrust and stretches is just ahead of the corner where the bright ring
occurs.On a bmg case it is about .050, my
wildcat 510HE .045, and a little less as cases get smaller..The 2/3 figure is based on that
thickness, leaving the id taking the pressure
and the brass holding part of the thrust,
with a little margin of safety.Using my math for my 12GA FH, where cases are made from bmg
brass, at 35,000 I got 11,900
lbs thrust.To section cases and do it
complicated; the id is .800 - (2 x .050)=.700.
Figuring id area x 35k = 13700 lbs thrust,
without the part the case holds.To get that
take section of the case, which is area
of .8 dia minus area of .7 dia. which gives
.130 sq in cross section x the tensile strength
of brass at that point about 25k and you have
3250 lbs thrust it holds subtracted from
13700 leaves 10,550 thrust.Now that amount of thrust that case holds is the same at 35k load
or a 65k load.My simple math gives higher thrust than actual at lower thrust levels, but that is good as it allows for safety in designing for lower pressure actions..
Using my math on bmg case with
65k load gives 22,186, and other way
sectioning cases, measuring insides,gives
22,230.Hope this is helpful.Ed.

Your number for the thickness of the brass at the yield ring is (most amusing!) just what I calculated would be needed in the .30-30. My intuition has indeed been proven wrong, and I yield the point (so to speak). My apologies to the ghost of Mr. Ackley.

Asdf-You were right if 30-30 was fired at
60k, but Ackley just did it at regular 30k
pressures needing less thickness... fun part is that he'd have got same results with regular cases as they would have gripped the sides of the chamber to hold 99% of what his
improved did.
Now that figure I gave for bmg case is the reason that all of bmg cases fired in sloppy mg chambers are stretched there.
IE the case grips the chamber with ten time
the force that the brass at the shiny ring holds.And bmg case sides has steeper angle
than 30-30 cases.

Also small cases like 30-30, the brass just above corner where stretching might
occur, will be stronger rated
per square inch, as it gets
work hardened more in manufacturing.Ie,
it has less radius and is closer to the
head area where the machinery work hardens the brass in the heading and pocket forming
operations..about 40k tensile strength.Ed.

quote:

Originally posted by asdf:
Bwana-be: if the case ruptures at maximum pressure, that pressure will immediately be applied to the entire diameter of the case. The volume opened by the fractured brass simply isn't enough to relieve the pressure. If the case fails early in the burn, you are correct, the escaping gases may prevent the pressure from reach its typical max.

Find a way to maintain 60kpsi inside the case, then - even maintaining that level of input - release the pressure out the sides and down a tube bigger than the case ID, and you will not see 60kpsi coming out of the tube. The input required doesn't change but the volume does. Another way to put it, throw a charge of powder that in an '06 would give 60kpsi into a 300 Win case and you get less pressure, because the case volume is greater. When the brass fails, you're effectively increasing the "case" volume to the chamber minus the brass....

Interesting topic...which begs the general question: Is there any truth to the recommendation that (All else equal!)..brass should Not be highly polished...just clean to keep it more tactile to adhere to chamber sides and thus reduce bolt thrust? Or is this really a
neglibible factor?
Comments from athe experts?
Thanks, Tom

Bwana-be: How about this argument? Say the case ruptures near max pressure in a .30-06. Now, the case capacity is about 69 gn of water, but at max pressure, the bullet's already part way down the bore, so all this hot propellant gas at 60 ksi occupies more than 70 gn water.

At rupture, the gases rush around past the fractured head, and start to fill the annular volume between the separated case head and the chamber walls. This annular volume is only a few thousands of an inch wide (SAAMI calls for a max difference in diameters of .011") and maybe .10" long, for a volume of about 0.2 gn water.

You surely must agree that once a volume of gas of 0.2 gn water has escaped past the case head, that the pressure in the 70+ gn chamber will not be greatly affected. For this reason, I say that at case separation, the OD area of the case can be briefly subjected to the max chamber pressure.

Bisonland-Super polished cases and chambers
will only reduce friction maybe 25%, but that still gives more grip on firing for
what the brass case holds of the
thrust.What the case holds of the thrust
is same, whether coefficient of friction
is .040, .030, etc.Ed.

I won't say "greatly," just proportionately!
;-)

quote:

Ackley improved cartridges may produce little or no bolt thrust.

This therory has been proved wrong in a recent experiment using very sophisticated apparatus.
The article was featured in PS Shooting Magazine called "Ackleys Mistake".

Tests were carried out with a standard 30-30 and the 30-30AI at 50kpsi. Both loads at this pressure developed the same identical back thrust.

Mr Ackley removed a lug on a 30-30 and fired low pressure loads to make his point.

Roy Wby turned the 300 H&H into an Imp and called it a 300 Wby. Straight min taper walls etc. and subsiquently revised his action and bolts to feature Nine locking lugs to withstand multi tons of pounds back thrust.

If the above statement had any validity why would he need all that much lock up beef.

Case head separations are the result of too much head space or a springy action. A case will cling to the chamber walls down to the pressure ring. Since the rear part of the cases or the case head is nearly solid brass it will not cling or expand to the chamber wall.

It only got one place to go under pressure and that is to the bolt face and it will stretch at the pressure ring. The more head space the sooner it will separate. In a Handi or the T/C if you don't load to nearly zero head space, your cases won't last very long.

In an AI bolt gun chamber loaded with zero head space cases last and last and last but not if you load to the plastic limit of the brass.
If you stay near the elastic limit 55-60 kpsi
You will be rewardeed with long case life.