Lug setback? What am I missing?

[adam32]

It's been well covered, no need for another person to try especially when someone is bound and determined not to understand!!

How many Lapua based wildcats (.25, 264, 270, .284, .30) do you know of that have encountered bolt lug setback? When being run at "normal" pressure.

I know it's impossible for you to agree with me on anything, but you have to see what I'm getting at...it's not just a simple PSI equation.

 

[codyadams]

No one in this thread ever said a lapua would have lug set back on a rem 700 action if ran at standard pressure. Only thing close to that stated was that the threshold, or room for error, for lug set back is lower with a Lapua sized case than smaller sized cases. And then it was explained how that is so with mathematical formulas that directly relate.

 

[jpndave]

One thing that seems to be a recurring problem here is that units/location/time keep getting dropped from the equation. 60k pounds/square inch inside the case does not translate to 60k pounds. The "per square inch" has to remain at the instant and location it belongs until accounted for.

 

[adam32]

No one in this thread ever said a lapua would have lug set back on a rem 700 action if ran at standard pressure. Only thing close to that stated was that the threshold, or room for error, for lug set back is lower with a Lapua sized case than smaller sized cases. And then it was explained how that is so with mathematical formulas that directly relate.

So a 270 Lapua will experience bolt lug setback at the exact same amount that a 338 Lapua will?

The fact that the overall Lapua case has 2-4x the capacity of other cases being mentioned has ZERO bearing on the problem?

Or rebate the rim on a Cheytac and run it in a Lapua Rem 700 action...it'll be safe since it's the same size case head, right?

 

[jpndave]

I'm not interested in engaging in a debate or "stupid" for the sake of arguing. I do enjoy a healthy discussion and appreciate learning from you all.

The bolt lugs don't care what cartridge is in front of them. It's about how much pressure gets applied. Several have posted correctly (J E CUSTOM on the last page and some others that I didn't go far enough back to find) that the pressure (and I think everyone has been posting peak internal pressures) is applied to the surface area co-planer (can't think of a better word for that right now) to the bolt face/lugs. Basically everything across the rear of the INSIDE of the case. It doesn't matter if it is a 0.585, 0.540, 0.470. 0.384 bolt face the total rearward energy and the peak pressure instant that could be transferred into the bolt head and therefore to the lugs is that pressure times the area just mentioned inside the case minus anything the brass takes away through resistance to the chamber walls. The previous numbers calculated have pretty much left the case friction out which is fine for discussions sake. The larger bolt faces on the same size action don't allow for any more lug bearing surface on the same action size. They simply will accommodate the larger case base which "usually" allows for more room inside the case to distribute a larger area of pressure which "usually" results in more overall total pressure pushing back on the bolt head and therefor the lugs. If you turned down a 338 Lapua 0.585 case to 0.384 is will still put the same amount of rearward force to the bolt head that it did as the larger head size all other things being equal. And no I not proposing that should be done or that the case would hold up. On the opposite end, if you were to put a 0.585 dia bolt in a 223 action, it would also take the same force from the 223 as it did with the 0.384 head.

If we want to use the bolt face diameter for load calculations then we need to use force number at the rear of the case, not a pressure measurement from inside the case.

All of the units of measurement have to be accounted for. We have a force, at an area, at a time, at a location. One of those can't be used without taking into account the others - or cancelling them out. It's math related to physics and it doesn't lie.

 

[MNbogboy]

So the force of dynamic recoil is not delivered to me and my stock through the bolt and lugs? Or is it possible that the "bolt force=area x psi" equation is just part of the energy imparted on the bolt? I'm not purposely being hard headed, but I can't imagine that the equal and opposite reaction of the kinetic muzzle energy does not impinge upon and transfer through the bolt face and lugs, somehow. I mean, that punishing pain of 8000 ft lbs of energy has to get back there by some mechanism. If it's not through the bolt, I'm at a loss. Sorry. Maybe I AM as dumb as my wife thinks.

Sounds like you are getting a grasp on it now.
The dynamic recoil is imparted by the weight of the whole rifle as a unit being propelled backwards away from the bullet. It is true that all the parts you mentioned lugs, bolt , stock etc are included in the said weight. Think of the rifle as a rocket engine the powder being the fuel that is why the amount of powder & the duration of its burn are included in the "recoil" formula.
Again the Static pressure against each part in the rifle is calculated in a different manner and technically these parts are treated as one part in recoil calculation.

Example: place a coffee can on the ground with a lit firecracker under it. Hold your hand on the can...bang...it will lift your hand and the can up.....
Now do the same but stand on the can....bang it probably won't lift your whole body weight up.
Treat the can as a chamber and the closed end of the can as your bolt face (the ground underneath is your bullet), in each case the Static pressure was the same against the can end. But the recoil was greater when there was less weight in the whole unit. The actual energy produced was the same.

 

[Johnny Jr.]

Q: 460 Weatherby 600 gr putting out 8000 ft lbs at 62000 psi
30-378 Weatherby 150 gr putting out 4300 ft lbs 62000 psi
identical interior case head diameter (stipulated in the wiki reference) and same psi. According to the formula, psi and head diameter are the only variables that dictate bolt forces. How does that extra 3700 ft lbs of energy bypass the lugs and get to my shoulder?

A: Recoil is not just an effect of the ignition in the chamber. Think about all the gasses and the projectile traveling down the barrel. Since they are moving away from you and you are holding a stationary object there are rearward linear forces imparted on the barrel that are then transferred to the recoil lug and action body.

Me: it's still hard for me to imagine that the 8000 and 4300 ft lb examples above impart the exact same force on the bolt, but......I'm not in a position to refute the WIKI explanation and scientific formula. Take a few minutes to read it just so everybody is viewing this from the same perspective. https://en.wikipedia.org/wiki/Bolt_thrust

 

[codyadams]

Or rebate the rim on a Cheytac and run it in a Lapua Rem 700 action...it'll be safe since it's the same size case head, right?

I personally have said that is in fact not the case, several times now, and that it is the inside case diameter and pressure that affect bolt thrust. Not exactly sure what your missing, but I am through repeating myself and will unfollow this thread now as it is diverting into eyes and ears closed conversation.

Carry on, be safe, and God bless America.

 

[adam32]

I personally have said that is in fact not the case, several times now, and that it is the inside case diameter and pressure that affect bolt thrust. Not exactly sure what your missing, but I am through repeating myself and will unfollow this thread now as it is diverting into eyes and ears closed conversation.

Carry on, be safe, and God bless America.

Exactly...in the very beginning I said that CASE diameter plays the role, not case HEAD diameter! The CASE HEAD has nothing to do with it! It's not a very hard concept, that's why the formula in the beginning of this thread that uses CASE HEAD diameter doesn't mean squat.

 

[Johnny Jr.]

Exactly...in the very beginning I said that CASE diameter plays the role, not case HEAD diameter! The CASE HEAD has nothing to do with it! It's not a very hard concept, that's why the formula in the beginning of this thread that uses CASE HEAD diameter doesn't mean squat.

I think some confusion can be attributed to a quote attributed to Kirby Allen, as stated in the very first post that started this thread

"2. the lugs feel MORE energy due to the bigger boltface"

if you read the wiki reference on Bolt Forces it clearly says it's the inside case diameter that matters, not the bolt face diameter. That should end the rebated rim questions. The wiki articles also says that using the case head diameter adds a safety margin to the calculations, but that's presumably for use on case heads that are relatively close to the body diameter.

 

[fiftybmg]

All very informative for me, making me stretch my brain. Thanks to everyone with patience.

Math equations are not wrong - complex systems are approximated with a number of them, not just one.

If it was such a simple thing to calculate, the interior ballistics textbooks would be very, very thin.

 

[MNbogboy]

Bottom line;
Chamber pressure and case body diameter determine bolt thrust. Bolt "thrust" is just an engineering term for the amount of force it experiences during the highest pressure instance of gas expansion during firing.

Muzzle energy is related to recoil.

Recoil is not dependant on bolt thrust and bolt thrust is not dependant on muzzle energy or recoil.
Comparing the two is apples & oranges.

The 223 Lapua w/55gr pictured above would have the same bolt thrust as a 338 Lapua w/300gr when both were run at the same pressure. But the 338 would also have far more recoil.

Bolt thrust will always be higher with the same pressure in larger diameter (piston) cases. These include the 338 Lapua, Norma and Bee along with a few others. Lug setback in Remington & Savage actions has been encountered because their original designs did not figure in these large diameter (piston) cartridges.

We don't all have to understand the engineering or mathematics determining this we just have to beware when assembling rifles.

Other ambiguities addressed;
Yes, greater volume is required in cases to drive heavier bullets at design pressures. Heavier bullets at pressure create more muzzle energy and thence recoil.
Bolt thrust however will be determined by the case diameter. Therefore longer cases with less diameter can have less bolt thrust while developing same pressures, muzzle energy & recoil.

That's it in a nutshell (my simplified version anyway...).

 

[Mram10us]

I might be missing it, but it doesn't seem that the bullet weight is being used in the equation(but for a few posts). Shouldn't this be a primary variable?

edit: Also, wouldn't recoil be a perfect measurement of the force applied to the lugs?

 

[adam32]

I might be missing it, but it doesn't seem that the bullet weight is being used in the equation(but for a few posts). Shouldn't this be a primary variable?

edit: Also, wouldn't recoil be a perfect measurement of the force applied to the lugs?

No because somehow the recoil goes AROUND the bolt lugs and gets distributed to the recoil lug and action body instead...that's one of the things you and I aren't smart enough to understand, but yet nobody can explain why.