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AR2201
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Loc: Oz
Backthrust on actions
      #37763 - 18/09/05 06:06 PM

Backthrust

Backthrust = Torque

Upon firing a cartridge backthrust acts to open the locked DR mechanism.

T = Force x Radius

Pressure = Force/Area

Assume the pressure of the cartridge expanding the case head against the face of the action translates the force acting to generate the torque.

Assume the same action ie. identical radius through which the force acts to generate torque.

Then the Torque generated by a 30-06 SAAMI 50,000CUP (head diameter 0.470") would be equivalent to a 45-70 (head size 0.505") at 43,309CUP or a 9.3x74R (head size 0.469") at 50,213CUP.

Am I on the right track?


AR2201
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500Nitro
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Re: Backthrust on actions [Re: AR2201]
      #37768 - 18/09/05 08:47 PM

AR2201

"Upon firing a cartridge backthrust acts to open the locked DR mechanism."

Where do you get this from ?

When a gun is fired the majority of the force is trying to move the barrels
away (forward) from the action - ie the force is a predominantly stretching

I admit that some of the force will act in a pivoting way trying to open
the gun but not much compared to the other force.


Why don't you get a DR or Shotgun, cut all of the locking bolts
and 3rd bites off and fire the gun. That should answer your question.


AR2201 - are you an engineer or a scientist by any chance ?
(Just asking from some of the questions you ask)

500 Nitro



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AR2201
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Re: Backthrust on actions [Re: 500Nitro]
      #37771 - 18/09/05 10:17 PM

500 Nitro

DRs barrels attach to the action on a pivot (open the action and the barrels swing up and away fron the action face), so any force acting to drive the barrels away from the action face will force them around that pivot, up and away from (off) the face.

Conjecture aside, the question really is about the term backthrust, this appears to be discussed as a phenomena distinct from pressure. I'm trying to figure out what this backthrust could be that is DR specific (the pivot action?) and referred to as both distinct from pressure and seems to be able to be calculated.

AR2201
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500Nitro
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Re: Backthrust on actions [Re: AR2201]
      #37773 - 18/09/05 10:38 PM

AR2201

"I'm trying to figure out what this backthrust could be that is DR
specific (the pivot action?) and referred to as both distinct from
pressure and seems to be able to be calculated."

Why is it DR Specific ?

Shotguns are made on the same actions so
why doesn't it or wouldn't it apply to them as well ?

While you are at it, do you think different actions
and thrid bites have an affec on this calculable force ?

ie Does a Webley Screw Grip action which used a Spade shaped
3rd bite have a different backthrust to a Webley PHV1 Action
which used a Club Shaped 3rd bite. Or is a Greener Cross
Bolt better ?

What about Self Opening actions. Does the Self Opening feature
affect the amount backthrust ? An example would be the Lancaster
"Wrist Breaker" action.

500 Nitro


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Otto
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Re: Backthrust on actions [Re: AR2201]
      #37774 - 18/09/05 10:54 PM

There's a great article on forces involved with firing a load in a SXS action in the NRA Gunsmithing book. Sadly, this book is no longer in print, but available at gun shows and, no doubt used book stores. For a SXS to close, the arc of the barrel breech requires the breech face to form an acute angle to the action flat. That acute angle actually makes the action want to stay closed when cartridge back thrust occurs, even if there was no locking mechanism!

Otto


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banzaibird
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Re: Backthrust on actions [Re: 500Nitro]
      #37775 - 18/09/05 10:55 PM

AR2201,

The way you seem to be using the term backthrust would make it the exact same as bolt thrust in bolt rifles. Thus the force is not specific to DR and as 500 Nitro pointed out is exactly the same as a shotgun built on the same style frame. In fact all cartridges have a "backthrust" as you call it when fired. The largest contributer to the amount of back thrust is probably cartridge design. Generally the more body taper to any case/cartridge the more "backthrust" it will exerpt on firing.

BB


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bonanza
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Re: Backthrust on actions [Re: banzaibird]
      #37776 - 18/09/05 11:41 PM

I think we have failed to mention here that the case expands in the chamber sealing it. If the gun is properly headspaced then in theory there would be no backthrust. If anyone here has mechanica or likewise software, do us a favor and produce a force vector analysis and end this debate. Now to the crux of the debate: why do some guns come loose with high pressure loads and some don't? I can't say - but every loose gun I have ever seen was dry as a bone, no grease on the pivot.

B.

--------------------


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500Nitro
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Re: Backthrust on actions [Re: bonanza]
      #37779 - 18/09/05 11:53 PM


"That acute angle actually makes the action want to stay closed
when cartridge back thrust occurs, even if there was no
locking mechanism!"

I tend to agree.

After all, if every time you fired the gun it forced the action open
all Double Guns would come loose a lot quicker than they do.

I agree about dry guns / pivots - it's amazing how many people
do not grease / lube the pivots and or locking lugs of Double Guns
they use.

Personally, you can produce as many force vector analysis's as you
like, I really couldn't give a s2#@t what forces are acting where on the gun,
as long as it works when I pull the trigger, shoots where I point it
and stops me from being trampled.

While you spend your time reading force vector analysis's and
working out backthrust and it's affect on Double Guns,
I'm going to spend my time getting ready to go hunting
with my Double Rifle.

500 Nitro


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banzaibird
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Re: Backthrust on actions [Re: bonanza]
      #37780 - 18/09/05 11:53 PM

The case seals the chamber from gas but that's about it. Even with perfect headspace you will still have backthrust. It just will be absorbed/dispersed in the tolerances and manner in which the gun was originally designed.

As far as shooting loose it could be any number of factors metallurgy, action design, cartridge design, load intensity or one of a 1,000 other reasons.

BB


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AR2201
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Re: Backthrust on actions [Re: 500Nitro]
      #37784 - 19/09/05 01:36 AM

500 Nitro

Shotguns are made on the same actions so
why doesn't it or wouldn't it apply to them as well ?

Funnily enough that is what drew my attention to it as someone (somewhere) was discussing building a DR on a shotgun chasis and had calculated (somehow) that the action could take the backthrust from the 450 BPE but not the NE cartridge.

My curiousity was piqued, I will have to find the original thread again to see if I can ask them what they were on about.

I know there is a 500 Nitro built on a Ruger U/O action on the market, it would be useful to assess if something built for a 12ga really would cope with a 500 Nitro.

Just a sec.

Nothing surprising here, 12ga SAAMI 11,500CUP this would be equivalent to a 500 Nitro at 21,335CUP or BPE levels. My you beaut program only cuts in with loads at 30,000, tepid stuff.

12ga are basically operating at BP pressures so the actions rating for a 12ga does not in and of itself indicate it as suitable for NE.

Even assuming it takes the SAAMI for the hottest steel shot 3.5" shells at 14,000CUP that converts to ~26,000CUP or trapdoor springfield levels.

500 Nitro in a trapdoor springfield, ah hem ... I think I will let someone else do that.

NB. this says nothing about the Ruger action's suitability, it is only commenting on the minimum strength action necessary to fire a 12ga shell at SAAMI intensity.

Though it does mesh with what the other bloke was on about, making a set of 450BPE barrels to go with his shotgun. Useful enough, a 350gn 0.458 slug at 2,150fps (there have been 45-70 sleeves available for 12ga for donkeys), but, I think the bloke was on a "see if I can do this" trajectory, good on him, I completely understand.

While you are at it, do you think different actions
and thrid bites have an affec on this calculable force ?

The force itself is a function of the cartridge so no. But, on how that force translates itself through the action, most definitely.

It is notable that dolls heads, cross bolts and other esoterica have been produced. There was a clear perception of need, whether there was a need is a very different question. Their merit, would depend entirely on the quality of production, because, unless the fit is such that each load bearing surface takes its share of the stress simultaneously, then one will bear the brunt and the others will be in effect useless.

My bet is these things were sales gimmicks. The Hyundai add where the snake oiled yobbo shuts the door, which gives a satisfyingly "solid" sound on shutting , and then says "solid" while looking into the camera springs to mind. What does the bloody door noise matter really?

What does a crossbolt matter? if things have gone so pear shaped that the massive main load bearers have given way what is a pissy crossbolt going to do for you?

It is interesting to check the load bearing surfaces to see what is actually doing anything. I had a crap shottie with a crossbolt that did precisely nothing, it didn't even touch the sides of its hole, shotgun worked fine.

Have any of yours got crossbolts? take a look and see if they are under load.

I would be very interested to see a Lancaster wrist breaking back lock action, apparently they were easy to open but a bastard to close, so struggled for popularity. Our unlamented snake oiled yobbo was clearly unable to work a pitch with that one.

AR2201
Those who can make us believe absurdities can make us commit atrocities



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500Nitro
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Re: Backthrust on actions [Re: AR2201]
      #37785 - 19/09/05 01:57 AM

AR2201

You were the one that said it is DR Specific and I was the one
who bought up Shotguns.

As to "Are Shotgun actions suitable for building DR's" ?
the answer is Yes they are.

Quite a few 577 Light Nitro / Nitro for Black Powder 577 3 2/4"
Double ifles have been built on Greener Shotgun Actions.

Marakai has one and he posted a link to his story about it the other day.

That gun which I sold him was built at the same time as
another one + a Cape Gun - 577 / 12 Gauge. There were
other DR's before this including 444 Marlin.

Ruger Red labels have been made in this country into
375 Fl Mag and 500 Nitro.

Re "I would be very interested to see a Lancaster wrist breaking back lock action, apparently they were easy
to open but a bastard to close, so struggled for popularity. Our unlamented snake oiled yobbo was clearly
unable to work a pitch with that one.

Sorry, Sold my Lancaster DR to a mate in the US and my Lancaster shotgun to a mate here.

As to "unable to work a pitch with that one"
If you are taking about "unable to work a pitch with Self Opener's"
What about all Purdey's and then the Holland Sidelock post approx 1922/23 when they
went from the AB Action to the Patented Self Opener ?

Finally, in regards to "apparently they were easy to open but a bastard to close"
do you really close your own gun. That's what loaders are for.
I strongly suggest you get yourself one, it makes shooting much more enjoyable.

I can recommend a few if you want but not sure which part of Oz you are from.


500 Nitro








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DUGABOY1
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Re: Backthrust on actions [Re: bonanza]
      #37791 - 19/09/05 04:55 AM

These egg-head strings come up about once a year, and they are always the same! Somebody takes a calculater, and crunches some numbers, and thinks he has the answer! Then the next guy has all the numbers in regard to the steel of the new, and that of the old double, so his formula is the difinitive formula. The next guy has all the different locking systems differentualted to show which is the strongest, so he has them rated, properly!

The fact is all these things apply, but none are the lone problem that causes the double rifle to come off face. Bonanza is right, the case expanding, the way the rifle is locked up, and the origenal fitting,also effect the equasion. one thing I have never seen one of the guys take into consideration is the weight of the barrel set the thrust is trying to move forward against the henge pin, and up against the under bites at the same time, while the brass case is trying to move back in the chamber with the walls of the case hugging the chamber walls, all in a nanosecond of time.

Gentlemen, all this happens in such a short time, and all similtaneously, that the movement is neglegable. The truth is the going off face in a well fitted double, is caused more by the crapy care most give their rifles. I would bet, if it could be proved, the a full 98% of the loosening of a double rifle action is due to wear, in the henge hook/pin mateing surfaces because of lack of cleaning, and proper lubracation. Even when over pressure loads are used, if the rifle fails, it is usually in the walls of the chambers, or shortly ahead of the chambers, not going off face! I hear the scare tactics in relation to the belted/rimless cartridges being the culprit causeing the OFF-FACE condition, and it contributes, but is not a significent contributer. IMO, the rimless rounds are more of a liability to the failier to extract, and/or eject, at a very inoppertune moment, than opening the action.

I guess what I'm saying with this long winded post is, take care of your double rifle, shoot what it was designed, and regulated for, and use proper care of the rifle, and you wont need any of the thories presented here, because each is meaningless on it's own, and collectively, they can only hurt the rifle if poor care is included in the mix!

RAVE ON, GENTLEMEN! Let me know what you find out at the end of this exercise! BYE!




--------------------
..........Mac >>>===(x)===>
DUGABOY1, and MacD37 founding member of DRSS www.doublerifleshooterssociety.com
"If I die today, I have had a life well spent, for I've been to see the Elephant, and smelled the smoke of Africa!"


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banzaibird
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Re: Backthrust on actions [Re: DUGABOY1]
      #37794 - 19/09/05 06:02 AM

Dugaboy,

I understand what you are saying. Everything you say is right, if you can afford a commercial DR. However the only double rifles that I have are all homebuilt jobs. That in mind the pressure stuff always interests me.

Eventually I plan on owning a nice DR that started it's life as a DR and not a shotgun but that won't be anytime soon. In fact I even know what I want, I just can't afford it at this time.

For example the $4,000-$5,000 for most low end doubles is approx the same I had into the first 4 DR I made. That was spread out over almost 9 years.

BB


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Marrakai
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Re: Backthrust on actions [Re: banzaibird]
      #37799 - 19/09/05 10:39 AM

I was determined to stay away from this monumental exercise in mole-hill mountaineering, but there is one thing that nobody seems prepared to mention. A double rifle built on an inadequate shotgun action may well go off the face when proof-loaded, due to the stretching of the action or deforming of the lumps, but is also likely to suffer catastrophic failure due to cracking the action at the point where the action-flats meet the standing breech. Such a failure certainly destroys the gun, and is very likely to result in injury to the shooter if it happens in the shooting field. Many BP shotguns in Britain have cracked the action when resubmitted for nitro proof.

A simple inspection with a caliper and some common sense will tell you whether a gun is at risk of failure due to small lumps or thin chambers, but only proof-firing will prove the action to be strong enough. This is a back-thrust issue, pure and simple! Nothing to do with torque!

Have a look in Austyn's 'British Sporting Guns..' for photos of cracked SxS actions. I believe Boothroyd has published a few too. Sobering stuff.

--------------------
Marrakai
When the bull drops, the bullshit stops!
--------------------------------
www.marrakai-adventure.com.au


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500grains
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Re: Backthrust on actions [Re: Marrakai]
      #37802 - 19/09/05 01:06 PM

There is a pretty good article on back thrust on the Lilja Barrels website.

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sportingrifle
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Re: Backthrust on actions [Re: Otto]
      #37870 - 20/09/05 04:54 PM

Hi all...I have that book. If anyone wants a reprint of this detailed article I would happily photocopy it and send you a copy. Email me your address at : kmaher80@hotmail.com. Also, AR2201, your elementary physics of the forces acting around the hinge pin are correct. The issue is complicated by other factors such as fitting tolerances, unequal bearing of different locking bits, etc but the jist of it is correct. Cheers Kevin.

"Any man who is willing to trade his liberty for a few pieces of gold, deserves neither and will probably lose both."


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unspellable
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Re: Backthrust on actions [Re: sportingrifle]
      #37894 - 21/09/05 02:53 AM

The main force tending towards opening the action is not back thrust but recoil. The line of the barrels is above the line of thrust into the shoulder, hence the gun rises in recoil. This force begins as soon as the bullet begins to move and reaches its peak when the bullet is only a few inches up the barrel. As the rifle rises, the muzzle end of the barrels lag behind causing the barrels to flex downward and produce a force on the action tending to open it. It is this effect that a third bite is intended to counteract.

As for putting rifle barrels on a shot gun action, pressure and back thrust are not the only considerations. recoil must be taken into account as well. I have an old Ithica Flues in 28 gauge. With modern loads this gun is limited more by recoil than pressure. Hence, I avoid 1 ounce loads in it even though they run the same pressure as a 3/4 ounce load.



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AR2201
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Re: Backthrust on actions [Re: sportingrifle]
      #39365 - 20/10/05 08:28 PM

All

Many thanks, good stuff (especially the greasing of hinges not oiling them). My calculations put the 45/70, 30-06 and 9.3x74R pretty much on a par as far as backthrust.

But, anyway I shuffle it, the 375 H&H Flanged comes out as a marked step up from these.

I note this seems to be reflected in manufacturers offerings with the 9.3x74R often being the top of the small/medium bore actions and the 375 the bottom of the big bore range eg. Merkel.

So we can say that a 45/70 405gn at 1850fps and a 45/90 405gn at 2050fps both generate the same backthrust (all else being equal) as they both are notionally operating at the same pressure with the same case head size?

AR2201
Rumours as to my death are greatly exaggerated


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500Nitro
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Re: Backthrust on actions [Re: AR2201]
      #39366 - 20/10/05 08:47 PM


AR2201,

What you have just stated English gun makers knew years ago !!!

No, relly, if you look at any 375 Flanged Magnum double by at least
Holland's and probably others - also ? 369 Purdey ?,
they use / have the reinforced action

That is the easiest way to tell if someone has rebored and not reproofed
a 375 2 1/2" Holland.

500 Nitro


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AR2201
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Re: Backthrust on actions [Re: 500grains]
      #39369 - 20/10/05 09:21 PM

500Grains

Thanks for the tip on the Lilja website, I checked their article on bolt thrust as you suggested and they use internal case diameter and pressure to calculate the backward thrust generated by a cartridge onto an action.

I had thought of this but as my database only lists external head diameters I have used them in lieu and stuck to relative statements.

But, the good news is, this means we can say that the 45/70 405gn at 1850fps and the 45/90 405gn at 2050fps generate the same back thrust (same pressure same case head).

AR2201
An investment in knowledge always pays the best interest.


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AR2201
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Re: Backthrust on actions [Re: 500Nitro]
      #39375 - 20/10/05 10:16 PM

500 Nitro

Ah yes, but now I know it.

AR2201
The greatest good you can do for another is not just share your riches, but to reveal to him his own.


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NitroXAdministrator
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Re: Backthrust on actions [Re: AR2201]
      #346245 - 28/10/20 04:53 AM

Quote:

Backthrust

Backthrust = Torque

Upon firing a cartridge backthrust acts to open the locked DR mechanism.

T = Force x Radius

Pressure = Force/Area

Assume the pressure of the cartridge expanding the case head against the face of the action translates the force acting to generate the torque.

Assume the same action ie. identical radius through which the force acts to generate torque.

Then the Torque generated by a 30-06 SAAMI 50,000CUP (head diameter 0.470") would be equivalent to a 45-70 (head size 0.505") at 43,309CUP or a 9.3x74R (head size 0.469") at 50,213CUP.

Am I on the right track?


AR2201
The sleep of reason breeds monsters






BTTT

--------------------
John aka NitroX

...
Govt get out of our lives NOW!
"I love the smell of cordite in the morning."
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crshelton
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Re: Backthrust on actions [Re: NitroX]
      #346263 - 28/10/20 02:13 PM

I like the explanations in a couple of books on this subject because they are presented in a more simple manner and the examples and equations work. KISS works better for me.
Oh well, time for bed.

--------------------
CRS,NRA Benefactor Member, TSRA, DRSS, DWWC, Whittington Center
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NitroXAdministrator
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Re: Backthrust on actions [Re: crshelton]
      #346269 - 28/10/20 03:03 PM

Quote:

I like the explanations in a couple of books on this subject because they are presented in a more simple manner and the examples and equations work. KISS works better for me.
Oh well, time for bed.




Yes please, need the KISS equation on back thrust. As a starter at least.

--------------------
John aka NitroX

...
Govt get out of our lives NOW!
"I love the smell of cordite in the morning."
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Hunter4752001
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Re: Backthrust on actions [Re: NitroX]
      #346271 - 28/10/20 05:47 PM

The rearward thrust, called bolt thrust in bolt actions, is the product of the pressure times the surface area on which it acts. In the case of cartridges, the force we are interested in is that acting in the rearwards direction. It is the product of the pressure times the surface area of the inside of the rear surface of the cartridge ie not the surface area of the external dimension of the case rim. eg a 45-70 may have a rim diameter of .608 but the internal diameter is considerably smaller. It become important when someone claims that because something like the Baikal MP221 can take 30-06 pressures therefore you can use similar pressure loads in the 45-70 variant OOOPS.

As detailed above the locking mechanism on doubles also involves other torquing factors which means the end result isn't so simple.


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