Lug setback? What am I missing?

[codyadams]

Assuming the above, what kind of chamber psi does it take to push a 230 at 3k psi from each cartridge?

I think you would find different numbers based on what powder you use. Due to the efficiency, burn rate and energy of certain powders, some will do it with less PEAK psi than others. I think it is done by maintaining pressure for longer, like a bell curve with a larger flat peak spot on the more efficienf powders. Just an educated guess....

 

[birdiemc]

But nm^2 would be a larger number not smaller. See my post above where 1psi =6895nm^2

Not gonna lie, I let google do the conversion for me

 

[birdiemc]

Not gonna lie, I let google do the conversion for me

Heres how going to mm^2 gets smaller numbers.....dang all you people making me think to hard tonight...lol

 

[cohunt]

Heres how going to mm^2 gets smaller numbers.....dang all you people making me think to hard tonight...lol

You missed my post--- I agreed with going smaller when using nmm^2 as I misread your units, I thought you were using nm^2

 

[adam32]

It's hard for me to make sense of it too.....if I hadn't seen the first hand result from other people, I would have a hard time believing it, honestly when I first thought of it without doing research, I figured bolt thrust would be LESS, due to being spread over a larger area.....however, the term "psi" somewhat helps me understand....as there is more surface area for the forces of expanding gasses to be imparted onto the bolt, thus being transferred to the supporting areas of said bolt, the lug abutments.

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.

 

[thebbc]

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[thebbc]

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[thebbc]

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[thebbc]

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[David Emerson]

Dan Lilja wrote a good article and it is on his barrel site about shear area on a 700 with the weatherby,rigby sized case. The problem is yes when you shear it is too late.

 

[thebbc]

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[Will Gray]

Think of the brass as a piston. The area of the brass (above the belt if belted: the area of the chamber) is the area of a piston to which the pressure in the chamber is applied. That area times the maximum pressure in the chamber yields the force on the bolt. The area and strength of the lugs (both on the bolt and in the action) must be large enough and of such material that the lugs are not damaged by even a gross overpressure in the cylinder. Good design would dictate an action several times stronger than the force of maximum loads. Shock loads are rough on steel so the action must be made of tough (not case hardened or brittle steel). I do not have the numbers but can be assured that creditable commercial gun manufacturers can tell you the kind of steel and hardness needed for safe actions.

 

[Opa-lopa]

Pressure x area = Load; if the case inside diameter is larger, there will be more load on the bolt. That said, for dry case & chamber & brass case, a considerable fraction of the aft thrust is shed to the chamber wall via friction, like nearly half. Lube the case, or get it wet, and the aft thrust the bolt must restrain will about double.

 

[Canadian Bushman]

Larger bolt face exerts more bolt thrust. Cohunt has it right.

This is why Kirby Allen for years tried to talk people out of building lapua boltface cartridges on .700 dia bolts. The barrel tennon dia also should really be brought up to 1.125 as well.

This is also why Shawn Carlock developed the 338 edge for factory remington actions and developed his terminator actions for the bigger boltface.

A particular quality of hardened steel is the harder it gets the closer the yield strength and tensile strength get to each other.

So in a sense, by the time the lugs set back enough for you to notice, they have already yielded. Once they yield, failure isnt far off.

With the big lapua boltface, the difference between safe operating pressures, and those that are high enough to yield the lugs can be as easy as a slight layer of oil left in the chamber or switching to a new lot of powder. So its really not a good idea to build a 338 lapua on an action with a 700 bolt.

PS. As the bolt dia increases in scale with the rest of the action. I.E. barrel tennon, lug dia, lug width etc.The action becomes significantly stronger with even a small increase in diameter.