Hi Clark,

Thanks for ringing in on this one. I actually read through the thread you linked up and found it fascinating. I'll have to rethink Lilja's assertion that lubricating cases have a significant effect on backthrust. Varmint Al's work appears to have dispelled that....but it seemed so intuitive...



I do still have some lingering questions and some clarifications for you. Regarding your point #2...In your debunking of the benefits of Ackley's reduced case taper increasing case/chamber adhesion, it appears you took measurements off a 30/30 and 30/30 AI for your trig calculations. A potential problem with that is Ackley's example cited was actually a 250/3000 and it's AI offspring. Ackley actually cites the 30/30 as an example of a fairly straight walled case elsewhere in the chapter. The stock 250/3000 appears to be a different case with much more significant taper to my uncalibrated eyeball. I don't have case dimensions for the AI version so I'm unable to redo your calculations, but I'm hoping you do.

Related to that, if the calculations once again show minimal differences, are there any other explainations for the results that Ackley observed when running up pressures without sticking the action (Savage 99)? For the record, I'm just along for the ride on this one...I thought Ackley's theories made sense, but I'm always open to being convinced of different explainations.



Then going backwards and trying to understand your point #1, "The breech face being locked would have stopped the brass' elastic stretch, and the locking mechanism could get beat up."

Not being intimately familiar with the M94 action, I thought that removing the locking lug as Ackley did, would allow the bolt to be driven rearward with minimal resistance offered by a "finger held" lever only. Did I miss something?



It appears that it was accepted by most in that thread that case/chamber adhesion significantly reduced bolt backthrust. According to Varmint Al's workup with the experimentally derived mu's, case/chamber friction reduced backthrust by slightly less than half. Varmint Al's Brass/416R SS coefficients of friction



Ricochet, thank you for giving perspective...but I liked the way you put it better in the other thread.

Quote:

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I still say that, just to be safe, you should plan for a "worst case" scenario of no brass friction resisting the bolt thrust. It shouldn't be dangerous to fire a rifle with an oiled cartridge. That's something that's going to happen a lot in the real world.

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Having found Lilja's Lug Shear article on his website, it appears that he did just that. His calculated shears were based on bolt thrust unmitigated by case adhesion (from Varmint Al's work, not applicable to oiled cases, but is certainly the case in case head separations - not exactly a rare occurance). Added to that margin is the customary 2 fold safety factor. Lilja on bolt lug strength In that other thread, someone mentions a safety factor of 3 to 8 times. I feel safer already.

Quote:

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After having overloaded 100 guns in 40 cartridges just to see what happens, I have seldom had a problem with breech face support. One exception was a Colt Agent Aluminum framed 38 special with steel cylinder. I went 2 gr. past the 357 mag max load for Blue Dot a 110 gr., and the frame bent. It still shoots, but is shows that the chamber, brass, or primer are not always the first to go. Bolt thrust is real, but not a big deal.

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Clark are you offering this as evidence that most firearms have more than sufficient breech strength designed in...or or you offering it as evidence that case/chamber friction is reducing backthrust significantly? I couldn't tell.



In any case, if you ever compile all of your "blow-up" experiments into a nice package, please let me know of the thread or site...I'd even pay good money for such a book!

I am sure the brass is getting traction, stetching, and subtractin force from the breech, because I can see the thin stretched parts.

I lubricate with moly bore creame on the cases to get max bolt thrust.



Looking at the Clymer web site under reamer design:

22-250 Ackley:

C = .468"

D = .455"

L = 1.344"



Slope = [.468 - .455]/1.344 = .000967

inv tan .000967 = .55 degrees

Cos .55 degrees = .999953224



22-250 Rem

C = .468"

D = .4156"

L = 1.31"



Slope = [.468 - .4156]/1.31 =.04

inv tan .04 = 2.29061 degrees

cos 2.29061 degrees = .999200959



ratio of the cosines = 1.000752866



What does it all mean?

Less that .1% change in traction.

With a 40 gr load in that case, the difference in traction would be like a change in load of .03 gr, or less than my scale can measure. Of course, the difference in case capacity betwen the cases is much greater.

The case does support some pressure, that is why low pressure loads with headspace will have protruding primers.
It would seem that the case stays forward until a presure of around 40 KPSI or so. If the case was oiled it will tend to expand back from the mouth and avoid "stretch marks".
When the case does come back it has the same thrust oiled or not. This is base on a finit analysis done by "OK Shooter", and my own work with pressure measuring tape.
The old timers used oiled cases to "proof" actions because that was the only way the low pressure cartridges would have any thrust at all. Note that Ackley did not try the 94 with a standard cartridge and no breech lock.
Finally as to the case taper; You either have to believe Trig functions or not (your choice)
Good luck!

irv,
After reading the thread that xxxx linked us up to, I've come to conclusions similar to what you're offering.

I'm curious about whether your and OK Shooter's work showed that once the 40K pressure threshhold was crossed, was the case exerting unmitigated backthrust in the amount calculated by F = P * A or as calculated by Varmint Al in his coefficient of friction page (case separation example)?

My only difficulty with the standard vs AI backthrust and Trig/Vector analysis approach is that I'm trying to reconcile Ackley's experiences that he wrote of with before/and after. Can you help me with that?

>>Some weak rifles, such as a Win model 94, will show primers backed out of the case with fairly stiff loads. The reason being the breach block is springing back as the pressures move the primer out.<<

Low pressure can cause backed out primers if the case to chamber headspace is a little sloppy. Look at a lot of .30/30 or .35 Rem. once fired factory brass. You will find many backed out primers and these loads are not high pressure. I have a very strong .30/30 (788 Remington) it produces some backed out primers with factory ammo. Loaded to headspace on the shoulder it does not back out primers with any load.
Ed

I believe that the events are; The primer drives the case forward, The case "sticks" due to friction, headspace allows the primer to protrude. If you question this fire an oiled case and the primer will no be protruding.
Good luck!

I have seen 22 Jet revolvers shoot allot more than 2 or 3 cylinders with no problems. The were hot loads that grabbed the walls.

By definition bolt thrust is the max pressure multiplied by the max inside area of the cartridge.

Now that force may or may not be the final force applied to the face of the bolt. Many terms will have to be included in the "FREE BODY DIAGRAM". Factors such as friction and headspace will all affect the final (mass X Acelleration) as seen on the face of the bolt. I think this is what we are refering to when we talk about breach thrust. In a perfect world, no friction or head space issues, bolt thrust would be exactly equal to breach thrust.

I hope this helps focus the discussion and clarifies the language that we are all throwing around.

Why isn't bolt thrust the force applied by the case to the bolt?. Seems like a good description to me.
Take Care!