Lug setback? What am I missing?

[GLTaylor]

Wish I had some popcorn. You guys left me so far in the dust I need something to do

 

[tribb]

see the calcs--- lets take a set pressure to simplify--62,000 psi chamber pressure--- that same pressure is exerted on a .470 bolt face vs a .585 bolt face --- same pressure exerted over a larger area equals more force, dont care how big the lugs are if they are equal in both guns the lugs in the larger bolt face will see higher forces

think of it this way--- 62000 psi is pounds per square inch so the amount of force is dependent upon the area it exerts the force over-- a .470" bolt face is .1735 square inches -- a .585 bolt face is .2712 square inches, soooo at 62000 pounds per square inch you would take 62000 x .1735 = 10757 pounds of force for the .470" bolt face---BUT 62000x .2712 would be 16814 pounds of force for the larger .585" bolt face---make sense now?

Yep. Same as hydraulic brake systems

 

[Labwhisper]

Yep. Same as hydraulic brake systems

Finally! After 4 + pages, the word "hydraulics" appears. Very simple concept, if you have a finite pressure but need to apply more force to do more work, increase the diameter (surface area) of the cylinder. In this case the bolt is the cylinder, the lugs resist the "work" the hydraulic pressure is trying to do, namely shear or move the lugs.

 

[5.56×250]

I just checked all Cohunts math and I checked mine again and figured out we said the same thing. There is more actual force exerted on the large bolt head than the smaller one.
Its because you are not placing a 60,000 lb
weight on the bolt, you are applying 60,000lbs to every square inch on the bolt...so back to your table example you're not placing a 1000lb weight on top of the 4ft square table, instead you're placing one 1000lb weight on every square inch of the table, which would effectively transfer 2,304,000lbs to the floor, vs a 1ft table which would only be 144,000lbs transferred to the floor.
4ft x 4ftx 144sq in per sq ft= 2,304,000
1ft x 1ft x 144sq in per sq ft= 144,000

I was thinking the same thing about both of you arguing the same point . I had to keep scrolling back up to the posters name to see which one of yall were trying to prove the same point . I was getting dizzy !!

 

[sp6x6]

I just remeber when I was talking with Glenn at Defiance,owner founder,and the look He gave me,when I said I had bolt lift on my 338NM.Then a stern warning. I mentioned Id done that in my 340 wby, and heard it was common practice in reloading. The short version was this is not that

 

[codyadams]

I feel that the issue you and others have seen first hand are more due to being overpressure than the bolt face diameter...if you run a RUM up to 80k+ psi you would end up with bolt lug set back also.

Yes, but you won't with a .223 bolt face. What is different.........

 

[adam32]

Yes, but you won't with a .223 bolt face. What is different.........

So if you rebate the case head on a Lapua you could safely run it up to 80k and not worry about bolt lug set back?

 

[cohunt]

So if you rebate the case head on a Lapua you could safely run it up to 80k and not worry about bolt lug set back?

WELLLLLL (it actually is based off the insdie of the case, not the outside)--- it is actually a very complicated equation/calculation due to manufacturing differences and specs
what we provided was the "easy" calc methods of bolt thrust--but if you read the wikipedia page I listed earlier the measurement is actually the inside diameter of the brass case, since each brass manu is different it is very hard to actually calc the true bolt thrust using these calculation methods, also the different brass alloys and state of softness/hardness will change the "case grip" to the side walls of the chamber thus actually changing the bolt thrust numbers (this is shown with NATO proof testing where they oil the proof rounds for testing to increase the bolt thrust numbers -- see the equation below for Ainternal, is actually the INSIDE area of the cartridge case head which will vary based on the "web" design


Formula

where:

• Fbolt = the amount of bolt thrust
• Pmax = the maximum (peak) chamber pressure of the firearms cartridge
• Ainternal = the inside area (of the cartridge case head) that the propellant deflagration gas pressure acts against

hows that for a mouthful

 

[codyadams]

So if you rebate the case head on a Lapua you could safely run it up to 80k and not worry about bolt lug set back?

Possibly, I don't know of anyone that has done it. However I do know that lug setback has happened on lapua bolt faces. Do you know of any cases of lug setback on a .223?

 

[bigedp51]

So if you rebate the case head on a Lapua you could safely run it up to 80k and not worry about bolt lug set back?

You do not measure the outside diameter of the case or its rim diameter to figure the force on the bolt face. Meaning it is the inside diameter of the case that exerts the force on the bolt face just like a hydraulic piston.

And any lube or oil in the chamber increases bolt thrust because the case is not gripping the chamber walls. Meaning a dry chamber and cartridge reduces the bolt thrust and this varies with the yield strength of the brass.

My 30-30 at 38,000 cup or 43,000 psi always has protruding primers because the chamber pressure is not great enough to make the case stretch to meet the bolt face. Meaning you can't use the math formula because only the primer contacts the bolt face.

Cartridge Pressure Standards
http://kwk.us/pressures.html

The British used the base crusher system of measuring chamber pressure and the copper crusher was located on the base of the case. And with this system, the cases had to be oiled to measure actual chamber pressure. Below is from the British 1929 Textbook of Small Arms. In this same book the Enfield rifle was proof tested using two oiled proof cartridges and if the headspace increased .003 or more the rifle failed proof testing due to lug setback.

Bottom line, the brass cartridge case acts like a cars shock absorber and effects the amount of force and its dwell time on the bolt thrust. Meaning the math formula being used to figure the bolt thrust is "ball park" depending on other factors.

And this is just like P.O. Ackley's experiment when he removed the locking bolt from his 30-30 and remotely fired the rifle and nothing happened. The case gripped the chamber walls and only the primer contacted the bolt face.

And yes larger diameter cases do exert more force on the bolt face but there is more to bolt thrust than a ball park math formula.

Below is from the H.P. White Laboratories.

 

[codyadams]

WELLLLLL (it actually is based off the insdie of the case, not the outside)--- it is actually a very complicated equation/calculation due to manufacturing differences and specs
what we provided was the "easy" calc methods of bolt thrust--but if you read the wikipedia page I listed earlier the measurement is actually the inside diameter of the brass case, since each brass manu is different it is very hard to actually calc the true bolt thrust using these calculation methods, also the different brass alloys and state of softness/hardness will change the "case grip" to the side walls of the chamber thus actually changing the bolt thrust numbers (this is shown with NATO proof testing where they oil the proof rounds for testing to increase the bolt thrust numbers -- see the equation below for Ainternal, is actually the INSIDE area of the cartridge case head which will vary based on the "web" design


Formula

where:

• Fbolt = the amount of bolt thrust
• Pmax = the maximum (peak) chamber pressure of the firearms cartridge
• Ainternal = the inside area (of the cartridge case head) that the propellant deflagration gas pressure acts against

View attachment 172620

hows that for a mouthful

This makes more sense. So I guess a rebated case would have the same or similar bolt thrust as a non rebated case I guess, simply due to the case size itself.

 

[adam32]

So nothing at all to do with the bolt face, but the casing itself?

Now you guys are starting to catch on...

 

[sedancowboy]

If you don't agree with our math then put forth your formula to prove that our math is wrong. See the funny thing about math is that it has no emotion. The Hydraulics analogy is valid. Most hydraulic systems operate at about 2750 psi. To get more work done you increase the surface area that the Hydraulic pressure acts on via a larger cylinder.
A good example of this is if we have a backhoe with a 4" cylinder on the bucket then the bucket will not likely rip apart. But if we put a 12" cylinder on a standard bucket it would more than likely rip the bucket apart. The psi of both cylinders would be the same but the amount of force on the bucket would be much more. Force is pressure doing work.

By the way all calculations, to be accurate, should use inside dia. and every case is different and would grip the walls of the chamber differently. Also how much lube if any is on the chamber walls or outside of the case. All these factors would effect the actual bolt thrust. We are simply using known areas of different case heads as an example the numbers may vary but the concept is the same. Larger dia. cartridges exert more force on the lugs than smaller dia ones.

To answer the question of the rebated rim the formula is the same. In your example the force on the case head would increase by the same amount of difference between the case dia and the case head dia. Minis the friction of the case griping the walls of the chamber. Without doing the math over which has been put forth multiple times lets say if you have a Lupua case with a rebated rim using your example of 80,000 psi in the chamber the the bolt face would experience somewhere in the neighborhood of 100,000 psi depending on how small the case head was reduced.

Stop and think a minute. The reason we have bigger pistons in big engines and bigger hydraulic pistons on bigger equipment is to get more work done via more force. To argue with the math is to deny that a bigger cylinder can do more work than a smaller one even though the pressure is the same in each cylinder.

 

[adam32]

Possibly, I don't know of anyone that has done it. However I do know that lug setback has happened on lapua bolt faces. Do you know of any cases of lug setback on a .223?

Nobody checks it, because it's a 55gr bullet at 3,000fps as opposed to a 300gr bullet at 3,000fps...think energy... they are running the same pressure but which one has more recoil? Recoil has nothing to do with bolt face diameter...