Lug setback? What am I missing?

jpndave said:

This is incorrect.

First the forces against the lugs contribute to some of the recoil. There are also other forces going on that push on the lugs not directly realized as recoil. For example, in taper walled shouldered cases chamber pressure is pushing forward on both the case taper and the shoulder. Those forces would cancel out a portion of the forces to the lugs as it pertains to recoil. However as it relates to lug pressure and potential setback (remember the original question for this thread?) the lugs take all of that pressure.

Second,

The force on lugs is the total of all the pressure exerted from firing the round with its relating time. See my previous post. It's the gas pressure and time less any factors taking any of that pressure such as case sticking to the chamber wall. Nothing except the pressure applies against the lugs. It's not all things just the pressure adding to but there are factors that may take away.

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jpndave said:

Are you really wanting a discussion here or just to antagonize and argue? If the former I'm in for the duration. If the later then I'm out on what could be a valuable discussion.

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jpndave said:

This is incorrect.

First the forces against the lugs contribute to some of the recoil. There are also other forces going on that push on the lugs not directly realized as recoil. For example, in taper walled shouldered cases chamber pressure is pushing forward on both the case taper and the shoulder. Those forces would cancel out a portion of the forces to the lugs as it pertains to recoil. However as it relates to lug pressure and potential setback (remember the original question for this thread?) the lugs take all of that pressure.

Second,

The force on lugs is the total of all the pressure exerted from firing the round with its relating time. See my previous post. It's the gas pressure and time less any factors taking any of that pressure such as case sticking to the chamber wall. Nothing except the pressure applies against the lugs. It's not all things just the pressure adding to but there are factors that may take away.

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jpndave said:

For the forces we are dealing with on the lugs the weight of the bullet does not matter. Think of a balloon. The pressure at any given point on the inside surface is the same as any other. A gas under pressure eqalizes against all the walls. If anyone doesn't understand this then they need to go back to physics 101. That's beyond what we are going to be able to cover here. I and I suspect many others here have made an assumption that we're all at a minimum level here and have a basic understanding of rifles and physics. If I have assumed incorrectly I apologize.

Force on the lugs is the pressure (we've been talking peak - so at that instant in time the pressure is highest) times the rear inside case area minus any other factors such as case adhering to the chamber wall. That's it. All that calculation does is tell us the peak instantaneous load on the lugs. The total force to the lugs is a combination of all the forces and the duration (time). The bullet weight would impact the time because it will take longer at a give force to move the bullet to the end of the barrel. For that the weight of the bullet would impact the cause we are using but not the calculations directly.

Recoil is the combined energy delivered rearward from all the forces related to firing the bullet. The force transferred to the lugs is one component of that. For that calculation the weight of the bullet does matter. As does the gas blast, weight of the rifle, length of the barrel, brake or not, etc.

Sorry for the wordy response and I hope I'm not being offensive here. Again if so I apologize.

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adam32 said:

I'm just stating what has already been posted by others.

We are all now in agreement that it is the CASE diameter and NOT the case HEAD diameter that contributes to bolt lug setback. That's been my point all along.

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sp6x6 said:

Thats why they say the weatherby Mark 5 is a strong action,9 lug,but also hear that its hard for all 9 to mate up properly

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MNbogboy said:

One more example; 338 Lapua...225 grain @ 3000....13# gun...approx 27 pounds recoil energy....
Now turn the barrel down and flute it...switch the stock to lightweight carbon fiber...switch the scope to a lightweight....now the same gun weighs 6-1/2#.....shoot the same load 225 grain at 3000 fps....approx recoil energy is now 54 pounds....
The calculated bolt thrust is the same in both cases. The only thing that has changed is the calculated barrel strength (which would "burst" much easier).
The point here is static strengths of materials are irrelavent to dynamic recoil calculations.
A pressure VALUE within a chamber/barrel is required to calculate the static strength of the bolt lugs and abutments.
The ACTUAL pressure within the chamber/barrel will propel the bullet forward and the rifle/components backwards.

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jpndave said:

For the forces we are dealing with on the lugs the weight of the bullet does not matter.

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jpndave said:

Not very clearly - it comes across as trying to pick a fight so if you're not intending to...

If we are going to get so picky on this you need to clarify that it is the INSIDE CASE DIAMETER.

Generally speaking the larger case heads are larger cases and more force. I suspect when Kirby made the statement that's being picked apart he was speaking in more general terms.

If I tell you pi is about 3, that would be correct. Or if you want to be a jerk or to specify more accurately you can say no you're wrong it's 3.1. no 3.14, no 3.141, no 3.1416, 3.14159 and on and on it goes.

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Mram10us said:

Offensive statements are things like "go back to physics101" We can have a nice discussion and ask questions without those kind of comments. I guarantee there are things you and I don't full understand that will only be remedied with good debate. We all have real jobs that probably teach us about these principles in different ways. Physics, aeronautics, ballistics, etc. Off my soap box ....

I would argue force on the lugs is definitely based on pressure in the chamber, BUT it is very much affected by the bullet moving down the barrel. Different burning powders obviously create pressure at different speeds allowing same psi in chamber BUT still moving that bullet at equal speeds. We are debating properly functioning rifles not the affects of a stuck bullet in the lands.

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Mram10us said:

Really like the example to show rifle weight contributes to the felt recoil.
Example: If we had a 230gr bullet being fired from a free recoiling 230gr rifle, what would that do to the forces on the bolt lugs? Would they be the same or more than the 100lb br rifle?

Edit: my calc teacher in college said to always use extremes when solving problems. Blame him

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jpndave said:

LOL, thanks for that I debated how to post and try to reel things back. Wasn't sure I got it right hence the apology in advance.

I agree and enjoy the healthy discussions. I've had to step back and think through lot of this as things go back and forth clarifying if not correcting my understanding.

Thanks for the reply mram.

Back to discussion,

The weight of the bullet (and friction engraving the lands) doesn't directly affect the lugs. It does affect how long the bullet is acted on by the gas pressure which raises the pressure/duration thereby increasing lug pressure. Just saying...

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MNbogboy said:

Forces on the bolt lugs would be the same with the same max chamber pressure. The velocity of the bullet will be a little more. This can be proved if you have a "good" known load with a low ES. Hold it real loose and shoot a few, then hold it super tight or have a big guy shoot it holding tight. You may see 25 fps or more difference.

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