Lug setback? What am I missing?

And for reference, here is Kirby's exact quote from another thread.

"When you look at max chamber pressure concerning the Remington M700, its really not that simple just to reply to a simple question.

If you take a Rem 700 chambered in 223, the receiver would take pretty much anything you could put to it, well over 100,000 psi. Certainly not saying it should be loaded extremely hot but the small case head of the 223 really does not tranpose alot of chamber pressure to the bolt face.

The larger the case head size, the lower the breaking point in chamber pressure will be for any receiver.

For example and this is just an example, suppose the Rem 700 will support 120,000 psi with a 223 Rem case head diameter. Jump up to the 30-06 case head size and that would likely drop to 100,000 psi.

Step up to the belted magnum case head size and that would likely drop to 85,000 psi. Step up to the Lapua class case head and that peak would drop to say 75,000 psi.

Its all a function of chamber pressure and the amount of area that the case head presses back against the bolt with.

Now remember all these numbers are strictly for example.

Where you can get into trouble though is that many that hand load for a rifle will start with load development and increase powder charges until they top out in pressure, or at least that they think is topping out in pressure.

If you have a properly accurized receiver, you will see far fewer pressure warning signs then with a factory rifle.

If your shooting a 300 RUM for example, you will get a hair over 65,000 psi and your primer pockets will start to loosen up dramatically giving you an OBVIOUS sign that your way over pressure.

The problem with the Lapua is that by the time you reach this high pressure sign, that being primer pockets loosening, you will be WAY over the safe working design levels of the Rem 700. A 338 Lapua case made by Lapua will take 70,000 psi and show no real signs of primer pockets loosening, especially with only one or two firings. Does this mean its safe.........

A Rem 700 will certainly handle a properly loaded 338 Lapua load. That being roughly 2700-2750 fps with a 300 gr SMK in a 26-27" barrel length. In a 30" barrel you will see around 2800-2850 fps. With loads in this range, the Rem 700 is more then strong enough to handle this level of load long term.

The problem is when someone that does not have a good amount of experience loads the Lapua up higher then it should be. In that case, it WILL stress the Rem 700. Will it come loose, no it will not but over time, it can stress the receiver dramatically.

I personally will not chamber a stainless Rem 700 for a Lapua class chamber. The reason again is not that the Rem 700 will not handle a properly loaded Lapua, even the Stainless will easily handle this level, its just in the event that someone pushes things to hard and do not realize what they are doing.

With a stainless receiver, if you push things to hard, you WILL set the bolt lug supports back in the receiver, especially the bottom lug support in the Rem 700 as its not supported by alot of steel. This happens to a greater degree then the chrome moly(blued) receivers simply because chrome moly is harder and will resist set back better then stainless.

This is not opinion, I have seen it actually happen. I put together a 7mm AM for a customer right when I came out with the wildcat. I had a Rem 700 personally in this same chambering and was more then happy with it. Mine was a chrome moly receiver, the customers was a stainless.

After several months, I got a call from the customer that he was having problems opening the bolt on a fired round. Told him to bring the rifle and load data and ammo he was using.

The rifle had a 26" Lilja 1-7 barrel and was loading the 200 gr ULD RBBT to a legit 3300 fps. Now in a 30" barrel, this would be about tops, in a 26" barrel, VERY HOT LOAD.

He said the rifle performed extremely well for around 75 rounds. After that, he started to notice that the bolt was getting hard to open. Not dramatically but noticable. He backed off his load to 3250 fps but still the problem was there and was getting worse until he had dropped his load to 3000 fps and could hardly open the bolt after firing a round. Total rounds down the bore was less then 150 rounds.

First off I told him his load was way to hot, at least his 3300 fps load. 3200 fps was about right. So why was his 3000 fps load doing this????

I could not figure it out so I told him to leave the rifle and the next day I took the rifle apart. It only took a few measurement from the receiver face to the bolt lug supports to tell me what was happening. From the receiver face to the bottom bolt lug support was a full 7 thou longer then the top measurement....... When I built the rifle, this measurement was less then 0.0002" difference....

Measured the bolt lugs, they were nearly identical in length as they should have been so the bolt lugs themselves were not compressing, only the steel behind them....

So why was the bolt locking up so tightly when fired. Well, the bolt lug support surface was compressing faster away from the cam ramp so when you chambered a round, the bolt lug would climb up the cam and onto the surface but when fired, the bolt would be forced back down the declined surface, effectively making a mechanical lock.

After I saw this, I measured some of his fired case with a 0.0005" dial indicater and it was clear what was happening and this was just more proof of it.

I wanted to test this more, so I took my personal rifle which had a 27" Lilja of the same twist and loaded it up to 3300 fps with this same bullet. I shot 100 round through the rifle at this level. And I can say for a fact that not one of the primer pockets loosened up any noticable amount in the Lapua cases. Certainly not a sign that the load was safe, only that the Lapua case is brutally strong.

After those 100 rounds which were on top of nearly 400 rounds down the bore I had already fired, I pulled the barrel and made the same measurements. In this case, the measurement from the receiver face to the bottom bolt lug support was 1.5 thou longer. It to had set back but but dramatically less then the stainless receiver....

From that point on, I made it a shop policy to never built a Rem 700 stainless receiver into a Lapua class chambered rifle.

Again, please do not read this and start saying that Kirby Allen does not believe the Rem 700 is strong enough to be chambered in the 338 Lapua, that is simply not the case.

All I am saying and I will say it again to be clear, if your using standard printed 338 Lapua load data, even top end printed load data, you will be fine with a properly built Rem 700, chrome moly or stainless.

IF you load hot loads thinking you can load to the limits of the 338 Lapua case in a Rem 700, you will definatly push the Rem 700 harder then it should ever be pushed to and far past its design limits.

Keep a head on your shoulder, do not get caught up in the "grass is always greener" theory, load to standard load levels and you will be fine and very happy with a very long life of your rifle. Push things to hard and you may see just how bad things can get if a Rem 700 or any factory receiver is mistreated.

As a side note, I fully believe that a properly built and accurized Rem 700 is as strong as any factory receiver on the market including the Wby MkV and Sako TRG-S or M995. Over load any of these and you will see the same results, and they will be bad. Load the Lapua properly, and you will have nothing but great results."
 
Last edited:
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...
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
 
Last edited:
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

But the actual PSI is the same, so the force on the lugs doesn't go up unless chamber pressure goes up. There's only 62k PSI of actual "force" on the entire bolt/action/lugs. It doesn't matter if the cartridge is a .223 rem or a .375 rum, the actual pressure on the entire action is the same. Otherwise you'd have a .223 rem running 90k PSI (if the brass could handle it)...
 
One of the big issues is lug surface area. On Lapua sized actions some manufactures left everything the same, but increased the bolt diameter which reduced the lug size. It's like putting fat tires on a truck so you don't sink in the mud. A .720-.730 size bolt with larger lug area like a Bat HR with larger threads is superior. If you think about it, the psi is same on a 6ppc and 338 Lapua, the Lapua is burning 70-80 grains more powder and the case has more area on action. Like a bus or a Prius both driving 60'mph and hit a wall.
 
And for reference, here is Kirby's exact quote from another thread.

"When you look at max chamber pressure concerning the Remington M700, its really not that simple just to reply to a simple question.

If you take a Rem 700 chambered in 223, the receiver would take pretty much anything you could put to it, well over 100,000 psi. Certainly not saying it should be loaded extremely hot but the small case head of the 223 really does not tranpose alot of chamber pressure to the bolt face.

The larger the case head size, the lower the breaking point in chamber pressure will be for any receiver.

For example and this is just an example, suppose the Rem 700 will support 120,000 psi with a 223 Rem case head diameter. Jump up to the 30-06 case head size and that would likely drop to 100,000 psi.

Step up to the belted magnum case head size and that would likely drop to 85,000 psi. Step up to the Lapua class case head and that peak would drop to say 75,000 psi.

Its all a function of chamber pressure and the amount of area that the case head presses back against the bolt with.

Now remember all these numbers are strictly for example.

Where you can get into trouble though is that many that hand load for a rifle will start with load development and increase powder charges until they top out in pressure, or at least that they think is topping out in pressure.

If you have a properly accurized receiver, you will see far fewer pressure warning signs then with a factory rifle.

If your shooting a 300 RUM for example, you will get a hair over 65,000 psi and your primer pockets will start to loosen up dramatically giving you an OBVIOUS sign that your way over pressure.

The problem with the Lapua is that by the time you reach this high pressure sign, that being primer pockets loosening, you will be WAY over the safe working design levels of the Rem 700. A 338 Lapua case made by Lapua will take 70,000 psi and show no real signs of primer pockets loosening, especially with only one or two firings. Does this mean its safe.........

A Rem 700 will certainly handle a properly loaded 338 Lapua load. That being roughly 2700-2750 fps with a 300 gr SMK in a 26-27" barrel length. In a 30" barrel you will see around 2800-2850 fps. With loads in this range, the Rem 700 is more then strong enough to handle this level of load long term.

The problem is when someone that does not have a good amount of experience loads the Lapua up higher then it should be. In that case, it WILL stress the Rem 700. Will it come loose, no it will not but over time, it can stress the receiver dramatically.

I personally will not chamber a stainless Rem 700 for a Lapua class chamber. The reason again is not that the Rem 700 will not handle a properly loaded Lapua, even the Stainless will easily handle this level, its just in the event that someone pushes things to hard and do not realize what they are doing.

With a stainless receiver, if you push things to hard, you WILL set the bolt lug supports back in the receiver, especially the bottom lug support in the Rem 700 as its not supported by alot of steel. This happens to a greater degree then the chrome moly(blued) receivers simply because chrome moly is harder and will resist set back better then stainless.

This is not opinion, I have seen it actually happen. I put together a 7mm AM for a customer right when I came out with the wildcat. I had a Rem 700 personally in this same chambering and was more then happy with it. Mine was a chrome moly receiver, the customers was a stainless.

After several months, I got a call from the customer that he was having problems opening the bolt on a fired round. Told him to bring the rifle and load data and ammo he was using.

The rifle had a 26" Lilja 1-7 barrel and was loading the 200 gr ULD RBBT to a legit 3300 fps. Now in a 30" barrel, this would be about tops, in a 26" barrel, VERY HOT LOAD.

He said the rifle performed extremely well for around 75 rounds. After that, he started to notice that the bolt was getting hard to open. Not dramatically but noticable. He backed off his load to 3250 fps but still the problem was there and was getting worse until he had dropped his load to 3000 fps and could hardly open the bolt after firing a round. Total rounds down the bore was less then 150 rounds.

First off I told him his load was way to hot, at least his 3300 fps load. 3200 fps was about right. So why was his 3000 fps load doing this????

I could not figure it out so I told him to leave the rifle and the next day I took the rifle apart. It only took a few measurement from the receiver face to the bolt lug supports to tell me what was happening. From the receiver face to the bottom bolt lug support was a full 7 thou longer then the top measurement....... When I built the rifle, this measurement was less then 0.0002" difference....

Measured the bolt lugs, they were nearly identical in length as they should have been so the bolt lugs themselves were not compressing, only the steel behind them....

So why was the bolt locking up so tightly when fired. Well, the bolt lug support surface was compressing faster away from the cam ramp so when you chambered a round, the bolt lug would climb up the cam and onto the surface but when fired, the bolt would be forced back down the declined surface, effectively making a mechanical lock.

After I saw this, I measured some of his fired case with a 0.0005" dial indicater and it was clear what was happening and this was just more proof of it.

I wanted to test this more, so I took my personal rifle which had a 27" Lilja of the same twist and loaded it up to 3300 fps with this same bullet. I shot 100 round through the rifle at this level. And I can say for a fact that not one of the primer pockets loosened up any noticable amount in the Lapua cases. Certainly not a sign that the load was safe, only that the Lapua case is brutally strong.

After those 100 rounds which were on top of nearly 400 rounds down the bore I had already fired, I pulled the barrel and made the same measurements. In this case, the measurement from the receiver face to the bottom bolt lug support was 1.5 thou longer. It to had set back but but dramatically less then the stainless receiver....

From that point on, I made it a shop policy to never built a Rem 700 stainless receiver into a Lapua class chambered rifle.

Again, please do not read this and start saying that Kirby Allen does not believe the Rem 700 is strong enough to be chambered in the 338 Lapua, that is simply not the case.

All I am saying and I will say it again to be clear, if your using standard printed 338 Lapua load data, even top end printed load data, you will be fine with a properly built Rem 700, chrome moly or stainless.

IF you load hot loads thinking you can load to the limits of the 338 Lapua case in a Rem 700, you will definatly push the Rem 700 harder then it should ever be pushed to and far past its design limits.

Keep a head on your shoulder, do not get caught up in the "grass is always greener" theory, load to standard load levels and you will be fine and very happy with a very long life of your rifle. Push things to hard and you may see just how bad things can get if a Rem 700 or any factory receiver is mistreated.

As a side note, I fully believe that a properly built and accurized Rem 700 is as strong as any factory receiver on the market including the Wby MkV and Sako TRG-S or M995. Over load any of these and you will see the same results, and they will be bad. Load the Lapua properly, and you will have nothing but great results."
Good write up. Thanks for taking the time to post this.
 
But the actual PSI is the same, so the force on the lugs doesn't go up unless chamber pressure goes up. There's only 62k PSI of actual "force" on the entire bolt/action/lugs. It doesn't matter if the cartridge is a .223 rem or a .375 rum, the actual pressure on the entire action is the same. Otherwise you'd have a .223 rem running 90k PSI (if the brass could handle it)...
false-- if the lugs were the same size for a .470 bolt head vs a .585 bolt head --lets say that the 2 bolt faces/lugs make up 1 square inch of area-- the .470 bolt head has 10757 pounds pushing on 1 square in so the lugs see 10757 PSI total force (or 5378.5 POUNDS per LUG)-- but the .585 bolt head has 16814 pounds pushing on 1 square inch so the psi on the bolt lugs are now 16814 PSI (or 8407 POUNDS PER LUG)-- the lugs do not see the same PSI as the bolt head does due to the different surface areas-- the chamber pressure is exerted on the bolt face which is transferred to the lugs but due to the differences in face area and lug area the PSI is not the same

That is why is is call PSI-- it is pounds per square in exerted NOT pounds-- when you increase the size that the PSI is pushing against, then you increase the actual pounds of force that is exerted

maybe think of it like this-- we have a 1000 pound object with a surface area of 10 square inches sitting on the floor===this equals 100 PSI of force on the floor, now take a 1000 pound object with a surface area of 1 square inch sitting on the floor== this same weight object would be exerting 1000 PSI on the floor rather than 100 PSI simply due to its surface area
--so invert this the other way, use a constant pressure of 1000 PSI not a constant weight--1000 PSI exerted over 10 square inches would be 10000 pounds of pressure-- BUT 1000 PSI exerted over 1 square inch would only be 1000 pounds of force--so in this case the floor would see 10000 pounds vs 1000 pounds of force
.
the issue here is people are using 32000 PSI (POUNDS PER SQUARE INCH) and 32000 pounds of force the same way and they are totally different ----even though a 223 runs at 55000 PSI, the bolt face only sees 6800 pounds of force due to the smaller .376" bolt face which would be about .112 square inches -- YET a 308w running at the same 55000 PSI chamber pressure would see about 10500 pounds of force on the bolt face due to the larger surface area of the 308 case

PSI is pressure, Pounds (or ft/pounds) is Force -- pressure and force are not the same thing

I really dont know how else to explain this so I guess I am out of this thread for now
 
Let me try one more time. Since the lugs did not change but the area of force on the bolt face did. Then there is a increase of force on the lugs. Image if you increased the area of the bolt face to a 50 cal would you expect the same lugs on a 223 to hold a 50 cal. Obviously no. Even though the pressure would be about the same.

Cohunt is exactly right and has done a very good job of explaining the difference in Force and Pressure. My hat is off to him. Sometimes we need to try and understand instead of arguing. Kirby Allen knows more about this and has more personal experience than most of us. After all his reputation depends on it.
 
Last edited:
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
Any force exerted on the boltface, is most certainly exerted on the lugs. If I have a 4' kitchen table with 1000lbs on it, or I have a 1' kitchen table, the legs feel the same force. Boltface area doesn't matter. Lug size matters.
 
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
After going back and re-reading, my initial post was talking about force exerted on the boltface and transferred to the bolt lugs. You are explaining chamber pressure. Helps if we compare apples to apples.
 
It is proven through first hand experience and observation by experienced smiths that lug set back happens with high pressure loads on a 700 action with a .585 bolt face. A simple search will bring up cases of this spoken of by reputable people about this fact. I have never once even heard about lug set back on a remington action with a .223 bolt face, reguardless of how high pressures are. The only difference is bolt face size, so the fact that the larger bolt face causes lug set back isn't really up for debate, it is a fact, proven with first hand experience. Bolt face size does matter.

I think the discussion should be more about the physics of how that actually happens. I myself don't know big enough words to explain it....
 
Any force exerted on the boltface, is most certainly exerted on the lugs. If I have a 4' kitchen table with 1000lbs on it, or I have a 1' kitchen table, the legs feel the same force. Boltface area doesn't matter. Lug size matters.
You are correct that a 1000lb weight on top of a table will transfer 1000lbs to the floor no matter the size of the table top. But that is weight not force. "Weight is a measure of the amount of force acting on an object due to the acceleration of gravity."
 
I accidentally sent before I meant to.
Since you are talking about force, force is measured in newtons. So take 60,000 psi, that equals ~414newtons/mm^2.
.585 bolt face has area of .269 square inches or 179mm^2
.470 bolt face has area of .173 square inches. 112mm^2
Force=pressure x area
414 x 179 = 74106 newtons applied to .585 bolt face

414 x 112 = 46368 newtons of force applied to .470 bolt face

Edited because I'm an idiot and cant even read what I typed...I came to the same conclusion as Cohunt
 
Last edited:
Warning! This thread is more than 5 years ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.
Top