Bolt thrust

[BigJ22]

A long time ago, a friend was having problems with pressure signs with a 6BR, bolt lift and and ejector hole prints on case heads, for the first couple of firings after cleaning. This stopped when he realized that the chamber swab he was using to "dry" his chamber was contaminated with oil, and he adopted a chamber drying method that worked. It always amazes me how people think that their untested imaginings are worth much. We all do it, but the degree to which people argue about things that they have not tested and have no hard data for is remarkable.

Interesting.

 

[DaveTooley]

Quick question, if I may: How did the chamber expand by 9 thou and not affect the tenon area of the action? Was the barrel tenon thinned out?

I'm sure there were subtle changes to the receiver and it may have enlarged the threads. Back then I wasn't chamber on my Haas so pitch diameter was hand fit to each action. I cut the tenon off and cut a new one to fit.

 

[Will Henry]

I once saw a Savage 110 (30/06) which had been fired with what appeared to be 58 grains of Red Dot (stored in a 4831 can) behind a 180 Sierra. The receiver ring had expanded by about .015" and the threads on the barrel had flared at the counterbore so removal was impossible. This was the earlier 110 which used a counterbored breeching system. Viewed from a few feet away, the rifle looked fine, but it wasn't!
A Remington 700 fired the same bullet loaded atop 49 grains of 4227 (substituted for 4320. I can't fathom the thought process which went into this.). This rifle suffered no major damage. The bolt nose expanded to seal the counterbore, as it was designed to do. I swaged the nose back down, replaced the extractor, checked headspace, and sent the owner on his way with some reloading advice.
The only lug failure I have personally seen was on a Savage 340. The owner had rechambered it for the 307 Winchester, which might have been OK , with suitable loads, but the single lug failed when fed loads at 308 Winchester levels. The owner was a well respected engineer who should have known better but who was notoriously reluctant to listen to any suggestions from a mere tradesman! WH

 

[Webster]

Why is it "bad"?
Will it cause lugs to fail? Or just increase head space?
If so how many rounds?

I'm just trying to get a handle on the physics, mechanics of this
No No.

The lugs should never fail unless you are shooting insane loads. You should clean and put a little grease on the lugs each time you shoot.

 

[jepp2]

A long time ago, a friend was having problems with pressure signs with a 6BR, bolt lift and and ejector hole prints on case heads, for the first couple of firings after cleaning. This stopped when he realized that the chamber swab he was using to "dry" his chamber was contaminated with oil,

I somewhat duplicated this. Known load in a Bat VR single shot action. Cleaned and over zealously used Hobo Lube on the fluted bolt body. First 2 shots normal, stiff bolt lift and extractors marks on the 3'rd and 4'th rounds fired. Scratched my head, stopped firing, took the rifle home and cleaned the single shot tray and chamber. Everything back to normal.

 

[jelenko]

I somewhat duplicated this. Known load in a Bat VR single shot action. Cleaned and over zealously used Hobo Lube on the fluted bolt body. First 2 shots normal, stiff bolt lift and extractors marks on the 3'rd and 4'th rounds fired. Scratched my head, stopped firing, took the rifle home and cleaned the single shot tray and chamber. Everything back to normal.

Note to self: Clean/dry the chamber.
In the past I have not cleaned/dried the chamber after the cleaning the bore. Especially after the last patch with oil on it.

 

[JSH]

What Boyd brought up I have seen on XP100’s during IHMSA matches, more than once. There was also an issue with case lube a fellow used.
I use a clean virgin over size patch for the bore, I tumble all the lube off of my cases after sizing, or dip in acetone.
I skipped cleaning 204 brass a couple of years ago, flat primers like a temp issue or hot load. Wiped cases off and cleaned chamber problem solved.

Some lubes/solvents migrate to places they don’t belong. TC Contenders would “auto eject” if the lock up got lube on it.

 

[SKWERLZ]

I’m not convinced you could get enough rifle powder in a case to shear Remington lugs….

It not the powder, more the pressure, load in some pistol powder and things get exciting.
That said I’m confident something else would fail before the lugs on a modern rifle.

 

[762willdo]

It not the powder, more the pressure, load in some pistol powder and things get exciting.
That said I’m confident something else would fail before the lugs on a modern rifle.

That’s why I specifically said rifle powder any idiot knows if you fill a .308 case with unique Bad things are going to happen….

 

[Eric Leonard]

for sure if you buy a used Farley made in the early 2000s call and make sure it went through the recall or have the heat treat checked.

 

[nksmfamjp]

This is a perfect example of how not to measure bolt thrust or excessive pressure. Know your signs of excessive pressure by measuring head diameter from the beginning with new brass. Find what is maximum for your rifle, .0008" head expansion is maximum! When you get to hard bolt lift you have gone past max and you are not going to save that brass without a considerable amount of needless work. Go to Hornady website for a clear explanation of how to measure excessive pressure. Sorry that is the Hodgdon web.site

As far as I can tell, this doesn’t work. It depends on how exactly the brass and chamber started. No 2 are the same. To me, the most reliable pressures are bolt lift which seems caused by case head flow into the ejector. I shoot a ladder. Where I find the first ejector mark, with magnification. I back down until they won’t occur, even in warm weather. About 1-2% reduction.

 

[BoydAllen]

As far as I can tell, this doesn’t work. It depends on how exactly the brass and chamber started. No 2 are the same. To me, the most reliable pressures are bolt lift which seems caused by case head flow into the ejector. I shoot a ladder. Where I find the first ejector mark, with magnification. I back down until they won’t occur, even in warm weather. About 1-2% reduction.

I have experienced bolt lift issues with hot loads,with an action that has no ejector.

 

[jackieschmidt]

for sure if you buy a used Farley made in the early 2000s call and make sure it went through the recall or have the heat treat checked.

I went through the “Farley Scare“, as I had three. The first two were very early circa ‘97 the third one was made in the early 2000’s.

In short, several shooters had Farley’s that the lugs just came off, and not as a result from any kind of over pressure.

Jim used S-7 Tool Steel for his bolts. This is a great choice as long as you adhere to the VERY stringent heat treating protocols required in achieving the exact mechanical properties you desire.

Jim was wanting 44 RC. However, some bolts came out with as high as 53+. He simply got careless in the tempering.

There was a recall. The idea was you send the bolt back, he would redraw the temper to the lower level if needed.

I had access to a RC hardness tester at Loan Star Heat Treating, so their lab man test mine, Pat Byrnes’, and Gene Bukys’. All were at 46 RC and below except one of mine. It was my favorite LV action. It these at 51.

I had to decide if I was going to send it back and get stuck in “Farley Time” and never see it again in a year, or live with it.

Since I had already fired thousands of rounds through it, with no I’ll affects, I decided to have the lugs magnafluxed for any fractures. It had none, so I decided to just shoot it. 20 years later it is still my favorite action in LV, and I lost count of the number of rounds i have put through it years ago. I even converted it to a drop port.

My other vintage Farley is the action on my 30BR VFS Rifle, which if I remember checked at 46RC.

Here is the RC mark left on my LV from 20+ years ago.

 

[Eric Leonard]

I went through the “Farley Scare“, as I had three. The first two were very early circa ‘97 the third one was made in the early 2000’s.

In short, several shooters had Farley’s that the lugs just came off, and not as a result from any kind of over pressure.

Jim used S-7 Tool Steel for his bolts. This is a great choice as long as you adhere to the VERY stringent heat treating protocols required in achieving the exact mechanical properties you desire.

Jim was wanting 44 RC. However, some bolts came out with as high as 53+. He simply got careless in the tempering.

There was a recall. The idea was you send the bolt back, he would redraw the temper to the lower level if needed.

I had access to a RC hardness tester at Loan Star Heat Treating, so their lab man test mine, Pat Byrnes’, and Gene Bukys’. All were at 46 RC and below except one of mine. It was my favorite LV action. It these at 51.

I had to decide if I was going to send it back and get stuck in “Farley Time” and never see it again in a year, or live with it.

Since I had already fired thousands of rounds through it, with no I’ll affects, I decided to have the lugs magnafluxed for any fractures. It had none, so I decided to just shoot it. 20 years later it is still my favorite action in LV, and I lost count of the number of rounds i have put through it years ago. I even converted it to a drop port.

My other vintage Farley is the action on my 30BR VFS Rifle, which if I remember checked at 46RC.

Here is the RC mark left on my LV from 20+ years ago.
View attachment 1417601

I’d had mine for 5 years and on the 8th barrel, probably 12-14000 rounds when it came apart.
Got lucky I guess as there was no injuries to me or anybody on the benches beside me.
It took about 4 months for them to get it back to me with a new bolt.

 

[Will Henry]

Nothing to do with bolt lugs but I recall making a splined shaft with an integral gear for a mill which was down. The shaft, made of s-7, came out nice and I had hardened it and set it on the bench while I waited for the furnace to cool down for drawing. I was cleaning up and organizing (my work habits make this an ongoing necessity) when I heard kind of a "tink" sound and saw something skitter across the bench. A closer look revealed that two of the gear teeth had jumped ship! Had to make a new shaft and was a little more careful with the heating and quenching on the next one. WH WH

 

[jackieschmidt]

I’d had mine for 5 years and on the 8th barrel, probably 12-14000 rounds when it came apart.
Got lucky I guess as there was no injuries to me or anybody on the benches beside me.
It took about 4 months for them to get it back to me with a new bolt.

On paper, S-7 looks to be the ideal steel to manufacture a bolt due to it’s high impact properties at a higher RC hardness than 4140.

The drawback is it has very stringent heat treating protocols not only in the temperatures but the time the piece is held at these temperatures.

Performed correctly, it’s great. Performed incorrectly, lugs fall off.

 

[VA_XTC_Shooter]

Correct. Here's the link previously posted. It explains bolt thrust in detail, along with how to calculate it, as well as examples of approximate case ID's of most cartridges, like 223, ppc, 308,magum, etc. It's by an engineer in the firearms industry...Dan Lilja. I'm sure most of us are familiar with him and his barrels.
I encourage everyone, again, to look at it. Very good info on this very subject.

A Look at Bolt Lug Strength - Lilja

All action designers are concerned primarily with producing a safe and functional product. All other considerations such as weight, finish, eye appeal and other cosmetics are secondary. From a safety standpoint, the strength of the bolt lugs is of prime concern. Bolt lug shear strength depends...

riflebarrels.com

I don't know the internal head diameter of 6.5 grendel or 6.8 SPC, anyone?
But, if I understand bolt thrust, the 6.8spc case can be loaded to a higher pressure in the AR platform compared to 6.5 larger case head due to bolt thrust, no?

 

[hpshooter]

i doubt the lugs would fail. More likely barrel would come apart but anything is possible.

In the mid 80's an experienced shooter and re loader in our area died as the result of a Weatherby bolt being driven into his skull. Our R&P club attempted to get copies of the State Police lab/accident reports to no avail.
Best information we ever obtained was that there was a reasonable chance the round was invertible charged with a pistol powder. If memory serves me correctly the Weatherby that was involved had 9 locking lugs. Vintage US Krags and Savage 340's have 1, I don't count the bolt handle as a lug like some do.

 

[gunsandgunsmithing]

I don't know the internal head diameter of 6.5 grendel or 6.8 SPC, anyone?
But, if I understand bolt thrust, the 6.8spc case can be loaded to a higher pressure in the AR platform compared to 6.5 larger case head due to bolt thrust, no?

Yes, a smaller case ID will generate less bolt thrust and can be loaded hotter, in this regard. I don't know the case ID of the spc either but smaller is smaller and generates less bolt thrust. The link I posted in the post you quoted me on does a great, much better than I, job of explaining it in detail and the formula for calculating thrust. It's similar to hydraulics. Just for clarity, OD or rim diameter doesn't count. If it did, rebating rims would decrease bolt thrust.