Lug setback? What am I missing?

[Mram10us]

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?

 

[WeiserBucks]

Tagging in for this discussion

 

[sedancowboy]

If I have done my math right you go from 10,560 PSI on a .473 bolt face at 60,000 PSI load pressure to 16,290 PSI on a .588 bolt face. So a 60% increase in pressure from a standard cartridge to a Lapua on the bolt face/lugs. I would think that is significant.

 

[cohunt]

i can't remember the term off the top of my head right now, but yes a larger bolt face/case head does exert more pressure on the bolt face/lugs

 

[cohunt]

got it-- bolt thrust is the term we are taking about

Bolt thrust - Wikipedia

en.wikipedia.org

check out the "quick calcs" at the bottom for the rifle cartridges to give you a quick idea of the increase due to bolt face/cartridge face increase in size

Stolle Panda Bolt Stress and Deflection Analysis

 

[sp6x6]

Mram,I replied in other thread. But as I said, i had some issues and I live next to Defiance,and spoke with Glenn in the past.Mentioned I had bolt stick, and he warned me not to go there with my 338NM.This was a in person meeting.

 

[Mram10us]

If I have done my math right you go from 10,560 PSI on a .473 bolt face at 60,000 PSI load pressure to 16,290 PSI on a .588 bolt face. So a 60% increase in pressure from a standard cartridge to a Lapua on the bolt face/lugs. I would think that is significant.

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)

 

[Mram10us]

got it-- bolt thrust is the term we are taking about

Bolt thrust - Wikipedia

en.wikipedia.org

check out the "quick cals" at the bottom for the rifle cartridges to give you a quick idea of the increase due to bolt face/cartridge face increase in size

Stolle Panda Bolt Stress and Deflection Analysis

Quick read that study, but they were accounting for .0002" of slop in one of the calculations. Again, haven't read the whole thing yet, so I'm probably missing something.

 

[Mram10us]

Mram,I replied in other thread. But as I said, i had some issues and I live next to Defiance,and spoke with Glenn in the past.Mentioned I had bolt stick, and he warned me not to go there with my 338NM.This was a in person meeting.

I would love to speak with him. Seems like simple physics(thrust vs weight). If the lugs and abatements were 1sqin v 1sqft, that would make sense. Assuming the same size lugs and abatements, how can it change?

 

[cohunt]

I would love to speak with him. Seems like simple physics(thrust vs weight). If the lugs and abatements were 1sqin v 1sqft, that would make sense. Assuming the same size lugs and abatements, how can it change?

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?

 

[Mram10us]

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

We are measuring the amount of pressure on the lugs, abatements and the face because they are all related, correct? Wouldn't this theory mean recoil would be different?

 

[cohunt]

We are measuring the amount of pressure on the lugs, abatements and the face because they are all related, correct? Wouldn't this theory mean recoil would be different?

force is exerted on the bolt face, not the lugs-- the force is then transferred to the lugs/face to contain this force/pressure-- you could do the lug force calculation also but the force on the lugs would be based off of what force is exerted on the bolt face first, and that comes from the chamber pressure which is relayed in pounds per square inch and a larger bolt face is a larger surface area therefore has a larger force exerted on it , not sure how else to explain it

 

[codyadams]

As far as showing pressure signs, that goes for ANY tough brass, not just lapua bolt face sized cartridges. Lapua, Peterson, Alpha, ADG, and so on, the tougher brass doesn't shot pressure signs until higher pressure as others, in general. For example -

Winchester .308 brass will generally show pressure signs, such as ejector mark, bolt lift, primer pocket stretch, ect. at a lower pressure (and usually velocity) than Lapua .308 brass will, due to softer brass. This doesn't mean both loads are at 64,000 psi, and the Lapua brass makes it go faster because it says Lapua and it cost more, it is because the Lapua brass is still functional and doesn't have the detrimental pressure signs such as ejector marks, heavy bolt lift, and primer pocket expansion that would be present on the same pressure load in the Winchester brass. The threshold for Winchester brass, for example, may be 62,000 psi before pressure signs appear, where as with the Lapua brass, you will be at say 69,000 psi before the same pressure signs appear. That is the Benefit of tougher brass, aside from better consistency, is being able to load higher pressure without the signs, or having it last much longer at standard pressure loads.

 

[adam32]

Wouldn't the larger surface area make the force/pressure on the face go down? Since it's "spread out"? Like a standard width track dozer compared to LGP tracks...