Lug setback? What am I missing?

[adam32]

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?

That "for every action there's an equal and opposite reaction" applies too...more recoil should also equate to more force on the bolt lugs...

Which brings up another question...will a rifle without a muzzle brake have more or less force on the bolt lugs?

 

[villagelightsmith]

ugh, the force is exerted in "pounds per square inch"-- when the size of the bolt face is increased then its "square inches"(area) goes up so the force goes up--its a fairly simple calculation but thats why I posted the Wikipedia link to show the actual factual data of the force exerted on the bolt face/lugs for different bolt face sizes/cartridges
the chamber pressure is not 62000 pounds, its 62000 pounds PER SQUARE INCHES-- so if you had a bolt face of 2 square inches the force would be 124000 pounds, but with a bolt face of only 1 square inch then the force is only 62000 pounds

So, we are talking about PSI, Pounds per Square Inch. Or better expressed as Pounds/Square Inch or P/Si.
The P remains the same, it's the Square Inches (Area or Square Inches) that are increased by the larger bolt face. Got it. Total pressure on the bolt face is dependant on the size of the bolt face. More surface area, more pressure.
So with more pressure against the face, the bolt lugs have to hold against the net pressure.
And one cannot really tell by normal means if the bolt has ever been over-ressured to the point of near failure.
Nor can one determine the true strength of a factory bolt without testing to destruction.
It becomes a matter of trust; you buy 'em and try 'em.

 

[Johnny Jr.]

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?

I am now really confused on the physics involved. So, I drastically reduce lug forces by rebating my 338LM to .473??Seems to me it would still kick like a mule, but be easy on the lugs. How can it recoil the same forces without going through the lug faces first?

 

[Labwhisper]

I am now really confused on the physics involved. So, I drastically reduce lug forces by rebating my 338LM to .473??Seems to me it would still kick like a mule, but be easy on the lugs. How can it recoil the same forces without going through the lug faces first?

It can't. The lugs hold the cork in the bottle. The lugs will resist movement depending on their particular mass and metallurgy.

 

[Johnny Jr.]

I didn't think so. This whole thread started with questioning the statement that "2. the lugs feel MORE energy due to the bigger boltface".

So lug forces are NOT a function of the boltface (or head diameter)? seems to me it's a function of how big a bomb you are setting off in the chamber.

 

[Gcan]

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 ase 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?

Ok. First, Recoil is a chain reaction. It begins at the bolt face and transfers thru the lugs into the abutments to the recoil lug into the stock and to your shoulder.

so a 7 lb gun that makes 120 ft lbs of recoil puts twice the force on the lugs that a 7lb gun that makes 60 ft lbs of recoil. Don't get confused between felt recoil which is dependent on rifle weight. Regardless of that weight, the initial energy transmitted into the lugs remains the same.


E=MC2 The question is how that energy is transmitted


i was attempting to keep it simple. The amount of powder determines the pulse length and available energy. Yes a heavier bullet and a lighter bullet at the same charge weight will result in lower chamber pressure on the lighter bullet and less thrust on the bolt lugs.

Lets go a different direction. A 50 bmg only makes 53,000 lbs pressure. Is there any doubt a 50 would Push the lugs on a .710 bolt right into the abutments and then shear them off? Its the available energy, the resistance to that energy, the area that energy is spread over and the structure resisting that energy that create the opportunity for set back.

now you could put 43 grains of H4350 in a 308 case and then put a 500 grain bullet on top of it and probably blow your gun up because chamber pressure would be 100k or so.

i think the point Kirby might be making is the available surface area on the lugs are such that when absorbing the energy of a 338LM launching a 300 grain bullet at 3000 fps will cause the lug to indent the abutment and (set back) before you might see dangerous pressure signs on the case. This could be a cumulative effect that happens after repeated below max pressure firings in a LM.

With most cartridges with the case lock up to the chamber wall, the rearward force of the case against the bolt are insufficient to dent the abutments before the brass shows pressure signs. I think Kirby might be saying that in the LM that might not be true. At some point some round will exceed or tempt the engineered limits of an action. How will those limits manifest? What happens when the case is more resistant than the action, as may be the case with the LM case. Personally, I suspect SS actions might yield to this before carbon actions. With the LM You have a huge case base that puts the max limit of pressure on the bolt lug faces to where they overcome the surface integrity of the abutments.

As I started, few people have Kirby's experience with max rounds in really big gun in almost all available actions. Id just take his opinions to heart.

 

[jarnold37]

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?

I too am missing something because my little brain is trying to understand this from a practical standpoint and not mathematical. I know, I just showed my lack of perspective. However, if the cartridge case has x lbs of rearward thrust, it seems that amount of thrust will be equal given that the bigger case head dissipates the thrust over a larger entire case head meaning that the result is the same rearward thrust only spread over a larger surface. The sum of that dissipated thrust is the same as a smaller case head with same thrust only concentrated in a smaller diameter. A 20 ton press punch, 3/4" diameter exerts the same energy as a 1" diameter punch only concentrated in a smaller area.

 

[Gcan]

as far as the hypothetical goes, I'm not sure you could get the case in the example to push a 3000 bullet at LM speeds before seeing pressure signs in the case. And if you went to a rebated head on a LM case we would see pressure signs long before we could get to max loads because all the case head pressure would be on a smaller area. The thing with the LM base is its got so much area to spread pressure across into the bolt that it may be one of the only current cases that can exceed the metal in an action's resistance to deformity before showing pressure signs.

 

[adam32]

as far as the hypothetical goes, I'm not sure you could get the case in the example to push a 3000 bullet at LM speeds before seeing pressure signs in the case. And if you went to a rebated head on a LM case we would see pressure signs long before we could get to max loads because all the case head pressure would be on a smaller area. The thing with the LM base is its got so much area to spread pressure across into the bolt that it may be one of the only current cases that can exceed the metal in an action's resistance to deformity before showing pressure signs.

Would a Lapua shooting a 55gr bullet at 65k psi and a Lapua shooting a 300gr bullet at 65k psi result in the same force on the bolt lugs?

 

[Gcan]

I too am missing something because my little brain is trying to understand this from a practical standpoint and not mathematical. I know, I just showed my lack of perspective. However, if the cartridge case has x lbs of rearward thrust, it seems that amount of thrust will be equal given that the bigger case head dissipates the thrust over a larger entire case head meaning that the result is the same rearward thrust only spread over a larger surface. The sum of that dissipated thrust is the same as a smaller case head with same thrust only concentrated in a smaller diameter. A 20 ton press punch, 3/4" diameter exerts the same energy as a 1" diameter punch only concentrated in a smaller area.

the issue needs to be rephrased. The issue is not equal thrust of this or that. Yes. Thrust is thrust pretty much. The issue arises with a case that can create enough rearward thrust to overcome the structural integrity of the lug abutments before the case shows pressure signs. The LM's bigger case head and thicker case base allows the same pressure to be spread over a larger area thus reducing the per sq inch forces. This creates the same pressure over a broader area allowing the fullest transmission of force to the bolt with the least deformity Of the case.

 

[J E Custom]

No one must have read the supplement to my post or you would get it. It has nothing to do with anything except the force placed on the lugs with everything being the same except the bolt face diameter. Has anyone ever heard of a 223 Setting the bolt lugs or the lug abutments back ?

It is simple Mater of physics. Another good example that was a problem with airplane design was the fact that sometimes a woman wearing stiletto's could punch a hole in the floor of the plane because of the small amount of surface contact with the total weight on them (Pounds Per Square Inch).

Just like the other part of the supplement addresses the oily chamber compared to the clean chamber. It doesn't have anything to do with PSI's but it does change the bolt loading.

We can think what we think, But physics will have the last word.

J E CUSTOM

 

[adam32]

Has anyone ever heard of a 223 Setting the bolt lugs or the lug abutments back ?

I've never heard of a .223 shoot a 300gr bullet at 3,000 fps either...

 

[adam32]

No one must have read the supplement to my post or you would get it. It has nothing to do with anything except the force placed on the lugs with everything being the same except the bolt face diameter.

Everything isn't the same though, the only constant is X amount of chamber PSI. Literally everything else is different. That's why I asked if you shot a 55gr bullet out of a Lapua at the same X chamber PSI will it have the same force applied to the bolt lugs as a Lapua shooting a 300gr bullet at the same chamber PSI?

Or better yet, no bullet at all?

 

[sedancowboy]

The LM's bigger case head and thicker case base allows the same pressure to be spread over a larger area thus reducing the per sq inch forces. This creates the same pressure over a broader area allowing the fullest transmission of force to the bolt with the least deformity Of the case.

I hope that is a typo because the pressure per sq in is the same over the larger area of the LM case head thus having more thrust on the lugs. If it is the same pressure it can't reduce it. It can add more force to the Lugs because of the larger surface area.

Really we are talking about lugs and abutments. The bottom lug abutment is the weak link in a Remington 700 action therefore as Kirby stated it was set back the most. Thus the 700 Action did not have the necessary safety margin to satisfy him when the The LM was used.