Lug setback? What am I missing?

[codyadams]

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

Now you guys are starting to catch on...

Casing diameter, according to the writing. Which unless a case is rimmed or rebated, is directly related to the bolt face size.

 

[adam32]

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.

That has nothing to do with the bolt lugs.

 

[adam32]

Casing diameter, according to the writing. Which unless a case is rimmed or rebated, is directly related to the bolt face size.

5 pages until someone finally mentioned casing diameter...

 

[codyadams]

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...

Professional gunsmiths and people with reputations for being knowledgeable in this field don't think twice about building a 338 edge on a 700 action, however question building a cartridge with a lapua bolt face, even with LESS performance, on the same action. Recoil is the same, energy is the same, the only difference is bolt face. Bolt face seems to matter to them, do you think it does not?

I don't want this to turn into an argument or become derogatory, in my opinion it is still just discussion. No offense is intended in anything I am saying. Just want to bring that up as this discussion continues.

 

[adam32]

Professional gunsmiths and people with reputations for being knowledgeable in this field don't think twice about building a 338 edge on a 700 action, however question building a cartridge with a lapua bolt face, even with LESS performance, on the same action. Recoil is the same, energy is the same, the only difference is bolt face. Bolt face seems to matter to them, do you think it does not?

I don't want this to turn into an argument or become derogatory, in my opinion it is still just discussion. No offense is intended in anything I am saying. Just want to bring that up as this discussion continues.

As you quoted Kirby, he's fine with a Lapua on a 700 if being run at the same pressures at a RUM/Edge.

If you take the formulas given in this thread, then a 223 at X amount of psi and a Lapua at the same psi rebated to a .223 case head would have the same amount of recoil also? Right?

 

[CA48]

Im pretty sure Shawn Carlock's 375 Terminator is based on a case that is 1/2" longer than the Chey Tac with more capacity and it has a rebated rim .590" to run in his terminator action.
I've read in the past where guys have tried rebating the lapua case down to magnum .535" and we're having Long term issues running them on Rem 700's. And where guys have rebated magnum cases down to standard .473 and we're having issues with the primer pockets opening up early without as much material there to support them.
Although Shawn designed his brass and action from the ground up I'm sure to work around these issues. Why would he do this if not to decrease bolt thrust?

 

[adam32]

I'm trying to wrap my head around how the bullet doesn't effect anything?

In other words, if you took a .223 and shot a 300gr bullet at X amount of PSI and then took a Lapua with .223 bolt face at the same PSI but shot a 55gr bullet. Which would exert more force on the bolt lugs? Or if you took a standard Lapua with a 55gr bullet and one with a 300gr bullet, which would exert more force on the bolt lugs?

 

[Gcan]

A member brought up reading about lug setback from Kirby Allen. He explained that a lapua sized boltface :
1. Doesn't show signs of over pressure till it's too late because of tougher brass and what I would assume would be the more area on the case head to absorb
2. the lugs feel MORE energy due to the bigger boltface

The first one, I will agree, makes a bit of sense since it's a bigger surface area, however, the ejector is still the same size. The second makes ZERO sense to me. What does boltface size have to do with lugs and lug abatements? The rearward force on the bolt should be the same regardless of bolt face size since the bolt lugs and abatements are the same size. What am I missing?

If Kirby says he believes it, I don't necessarily have to fully understand it to accept it. Kinda like brain surgery. In this case he is 100% correct.

I could make several arguments but Clearly the lager bolt face puts more pressure into the lugs and abutments.


we all think in terms of equal chamber pressure being equal and that is deceiving.

simply put, if you know anything about air cylinders then Ill use 125 psi in this example. Two cylinders.

1" piston @ 125psi = .7845 sq/in x 125lbs = 98 lbs force on piston.

4" piston @ 125 psi = 12.466 sq/in x 125 lbs = 1558 lbs of force on piston.

Both pistons are operating at 125psi.

Now do the same equations for a 223 at 63,000 psi and a 338 at 63,000 and remember the bolt OD may be identical

Now consider the added thrust on:

The web thickness between the face and the end of the firing pin might be a factor. After all, regardless of the lug size, its whats under them that matters most. Then you have the same size or larger extractor as cases size increases. So in the same bolt body diameter, as case size increases the bolt face/lug area becomes proportionally weaker when, in a perfect world we would want the bolt to be proportionally bigger.

simply put: a 6mm anything at 63,000 psi does not put nearly the same pressure on bolt lugs that an edge or LM might at same chamber pressure.

 

[codyadams]

I'm trying to wrap my head around how the bullet doesn't effect anything?

In other words, if you took a .223 and shot a 300gr bullet at X amount of PSI and then took a Lapua with .223 bolt face at the same PSI but shot a 55gr bullet. Which would exert more force on the bolt lugs? Or if you took a standard Lapua with a 55gr bullet and one with a 300gr bullet, which would exert more force on the bolt lugs?

That is an interesting thought. It is obvious which would have more recoil, but bolt thrust I am not sure is related, I think it (bolt thrust) is more related to pressure, and the case (or rim) diameter....the recoil (I believe) is generated and varries based on the speed of the bullet as it travels down the bore, bullet weight, as well as the muzzle blast, which with more powder, especially unburned powder, creates more muzzleblast.

Another theoretical thought, if you got a case with the diameter of a Lapua, but made it so short that the total case capacity was that of say a .308 win, and loaded it to say 65,000 psi and shot a .338 caliber 300 grain bullet out of it (would obviously be slow) and then shot the same bullet out of a .338 Lapua at 65,000 psi, would the bolt thrust differ between the two, or remain the same?

 

[J E Custom]

Example:
we have 2 identical actions shooting the identical load, except the boltfaces. Wont the lugs and lug abatements see the same amount of pressure? Case and chamber walls account for very little (need another thread for that topic)

The larger the bolt face the higher the bolt thrust with the same pressure, It is like a piston in an engine, the larger the piston the higher the load on the rod and crankshaft. (More power) so the more robust the rods and crankshaft have to be.

I personally don't recommend a 700 Remington action for anything larger than a standard magnum bolt face for this and other reasons.

Just my opinion

J E CUSTOM

 

[Mram10us]

The larger the bolt face the higher the bolt thrust with the same pressure, It is like a piston in an engine, the larger the piston the higher the load on the rod and crankshaft. (More power) so the more robust the rods and crankshaft have to be.

I personally don't recommend a 700 Remington action for anything larger than a standard magnum bolt face for this and other reasons.

Just my opinion

J E CUSTOM

Roger. Back to my question of same bullet at same speed. Wouldn't the lugs Feel the same force?

 

[J E Custom]

Roger. Back to my question of same bullet at same speed. Wouldn't the lugs Feel the same force?

No, The difference is bolt thrust. If you shot a load with the same bullet weight and caliber, that produced the same pressure, with the same powder burn rate, the load would be greater on the same size lugs and abutments with the larger bolt face than a smaller bolt face.

This might help clear this and other issues up on things that effect bolt thrust and lug set back.

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J E CUSTOM

 

[Mram10us]

No, The difference is bolt thrust. If you shot a load with the same bullet weight and caliber, that produced the same pressure, with the same powder burn rate, the load would be greater on the same size lugs and abutments with the larger bolt face than a smaller bolt face.

This might help clear this and other issues up on things that effect bolt thrust and lug set back.

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J E CUSTOM

Pressure, yes, but doesn't it take the same amount of force to move the bullet the same speed?

 

[blackbrush1]

Harder brass flows less...does it therefore produce lower bolt thrust?

A less tapered case (ie Ackley Improved) produces less bolt thrust does it not?