Concept of 'bolt thrust'

[SRMohawk]

Gentlemen,
I posted an inquiry here regarding short barrel life several weeks ago because I've been perplexed as to why the previously exceptional accuracy of two different custom rifles I own suddenly deteriorated after having spent only 600-650 rounds with them. Of course, I rebarreled one (a .338 Lapua) and it promptly returned the performance I was used to getting out of it. But recently, my .300 WSM has experienced the same fate. No one has been able to explain definitively why this has happened either.

But tonight, I was doing some Internet research on the subject and found an interesting article that briefly referred to a concept with which I am completely unfamiliar -- 'bolt thrust'. Unfortunately, this article did not elaborate in any great detail, but did say that this phenomenon occurs occurs to a much greater extent in rifles chambered for the short magnums (which says little about why my .338 Lapua chewed a barrel up in less than 650 rounds) and that it's NOT conducive to accuracy.

Are any of you familiar with this concept? If so, please elaborate for me!

SRM

 

[Kamicosmos]

First I've heard of 'bolt thrust'. Got a link? I don't see how the bolt can in any way affect the condition of the barrel's bore...

 

[Vex]

Might have something to do with headspacing? Short magnums are severely bottlenecked.

 

[ugaarguy]

I don't know about "bolt thrust" but I'll venture a guess about the problems. Perhaps the throats wear quickly from the high pressure and high velocity of those cartridges which could then compound even minute amounts of headspace problems (which could again happen more quickly with the higher cartridge pressures).

 

[Ol` Joe]

Here is a good explaination

http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

 

[cracked butt]

Unfortunately, this article did not elaborate in any great detail, but did say that this phenomenon occurs occurs to a much greater extent in rifles chambered for the short magnums

Just from context, what I'm thinking is that short magnums burn alot of powder compared to the surface area of the brass. When a cartridge is fired, the brass expands to fill the chamber, it also grips the chamber very tightly so that the force generated is absorbed by the chamber walls through friction instead of it all being put on the bolt face and consequently on the locking lugs. The brass springs back when the pressure is released and only grips the chamber for milliseconds. My wild scientific guess is that short magnums have less brass to grip with in relation to the amount of energy they contain and put a larger proportion of force on the bolt (bolt thrust) than a non magnum cartridge. Over time, force put on the boltface will increase headspace, either very slowly or more quickly depending on the pressures of the cartridges fired and how much pressure the bolt takes with each firing. What does extra headspace mean? it means thet brass needs to stretch more to fill the chamber giving you less consistant shot to shot velocities. If you reload, you might want to try to compensate for increased headspace by neck sizing only using a Redding bushing die or a Lee collet die or avoid bumping the shoulder back any more than necessary if you are using a standard FL sizing die.

 

[Drue]

Bolt thrust is the total force placed on the bolt when firing. It's the pressure developed in the chamber at firing times the area of the case head. For a given operating pressure, the larger the case head the more pounds of thrust on the bolt.

If the thrust on the bolt exceeds the strength of the action the locking lugs can be battered back into the locking shoulders gradually increasing the headspace. If the action it correctly made and of normal strength there should be pressure signs and probably case failures before this starts to happen. The action would usually become more difficult to operate if this were happening.

Magnums and other rounds that use a lot of powder tend to burn out barrel throats faster than others but 650 rounds is not that many. I think that I would use a borescope or a throat erosion gauge to check the barrel on the .300. Then a headspace gauge to see if there is any setback in the bolt.

Drue

 

[xring44]

Failure to remove all the copper fouling in cleaning leads to poor accuracy, since I don't know what your cleaning regiment consists of this is a possible explanation for short barrel life accuracy. Case shape does have a bearing on life expectancy, the radically sharp shoulders and short necks are a combination that contributes to early loss of accuracy. The rate of fire also is a major factor, three shots between allowing the barrel to cool is plenty.
Bolt thrust was well explained in the above posts but will mention that you can cause this to accelerate if you bump back the shoulder in resizeing.

 

[dfariswheel]

Proper bore cleaning IS a consideration.

When a top custom barrel maker gets a barrel back from a customer with complaints of loss of accuracy, the FIRST thing they do is give it a good cleaning.

According to one maker I talked to years ago, the majority of these "inaccurate" barrels were simply fouled, and cleaning restored them to top accuracy.

About the only time I've heard of valid cases of a barrel loosing accuracy at very low round counts is with super-hot rounds like the infamous early .17 calibers.

 

[HankB]

Considering the difference in tensile & yield strength between cartridge brass and the heat treated alloy steel going into a quality bolt action rifle, I have a hard time accepting the "case grips the chamber walls" idea of reducing bolt thrust, even though it's been repeated many times. (If anyone can cite actual measurements from a ballistic laboratory showing this effect is anything other than minor, I'll revise my opinion, but until then I regard it as hooey, no matter who's saying it. Show me the data!)

In any case, all else being equal, short fat cartridges are supposed to have an accuracy advantage over long skinny cartridges. If case volumes are the same, velocity, chamber pressure, and load data should be pretty much the same, and the fatter cartridge will produce more "bolt thrust." Yet it will likely be more accurate, thanks to more uniform burning of the short, fat powder column.

So unless "bolt thrust" is high enough to damage the rifle (which means excessive chamber pressure) I don't see it as having an effect on accuracy.

 

[ClarkEMyers]

Neal Knox had a rail gun in .220 Swift that burned up the throat pretty fast.

Neal Knox had a rail gun in .220 Swift that burned up the throat pretty fast. I suppose he was using high loading density, as is common in benchrest, but it's been years since I thought about it. Somebody with the Handloader CD can look up the facts.

My memory is that Neal used a borescope regularly given that the whole thing was an experiment and for something to write about. IIRC the origin of the rifling was effectively moved 3 inches down the throat at well under 1000 shots. His conclusion was that Swift could shoot benchrest but not for very long. Accuracy was still sufficient for varminting when he pulled the barrel off the rail gun.

The finite element analysis is impressive and points out that pressure will peak at some places at well over average pressure values. I am reminded of tales that the way to get all 9 lugs of a Weatherby Mark V action to bear is to fire bluepills until they do. I wonder if the bolt face isn't going out of square as the lugs bear unevenly - maybe alternating from side to side - rather than a headspace issue?

I have no data on case grip and I have no recollection of any controlled tests. I do remember tales of experiments with lubricated cases and some sort of crusher indicator on the bolt face or as appropriate to the action - I think some of the early or maybe European proof houses used lubricated cases? Certainly there are stories of the .22 Jet and other cartridges working best with very dry chamber walls - not a controlled test at all. See also fluted chambers to sometimes float the case and sometimes restrain the case

 

[mete]

Fluted chambers as in HK G3 type delayed blowback guns ,are there because the bolt starts to move at the peak pressure .The fluting prevents the case from grabbing the chamber tightly and the head tearing off....

 

[Zak Smith]

Whose barrel was in that 338, and what specific loads were you shooting?

What was the original, and final accuracy (after deterioration)?

650 rounds is WAY less than one should expect from a 338 barrel.

 

[Bill T]

The bolt thrust exerted on the bolt itself is in direct proportion to the rear surface area of the cartridge in question. Short Fat Mags develope more bolt thrust simply because they have a larger surface area that contacts the bolt face of the rifle. A .223 WSSM for example has over twice the rear surface area of a standard .223 / 5.56 MM round. This in itself has nothing to do with cooking off a barrel. I have a .338-378 Weatherby Magnum in a Weatherby Accumark. This gun is fast going past the 700 round mark and the bore is fine. My primary go to powder with this gun is WC-872 Military Surplus Ball Powder. It consumes over 110 grains of the stuff per shot with the Sierra 300 Grain Matchking .338 bullet. The key with these large overbore Magnums is not letting the bore get too hot. If it's too hot to touch, it's too hot to shoot. This limits me to 3 shot groups only, accompanied with a long cool down process. The less you cycle a barrel from hot to cool, the less heat checking you'll get. Heat checking can best be described as small surface cracking on the inside of the barrel steel just ahead of the chamber / throat area. It requires a bore scope to see. Even if a bore becomes heat checked it can sometimes be saved by firelapping. David Tubb produces an excellent kit for this purpose. It comes with different grade abrasive bullets you handload then shoot. This process smoothens out the barrel and can restore accuracy to sometimes new levels. But as I said the best medicene is to avoid heat as much as possible. Bill T.

 

[Art Eatman]

I never heard that term, "bolt thrust". Howsomever, the force acting rearward on the bolt is the chamber pressure in pounds per square inch multiplied by the cross-sectional area of the inside of the rear of the cartridge case. The outside diameter, minus twice the wall thickness, and apply the good old Pi R-Square formula. (No, Art, pie are round; cake are square.) That gives the force, in pounds, acting to the rear.

This force is resisted by the lugs of the bolt and the steel of their slots in the receiver. You need to then know the shear strength of the lugs and their cross-sectional area to calculate the ability to resist this rearward thrust.

Lemme play: Guesstimating the ID of an '06 case at 0.35" and a chamber pressure of 50K psi. The cross-sectional area is about 0.096 square inches. Times 50K gives a force of 4,800 pounds acting on the two lugs. 2,400 pounds per lug.

If the yield strength in shear of the hardened steel is 125,000 pounds per square inch, you'd then need 0.019 square inches per lug. The lugs on my Rem 700 measure about 0.45 x 0.45, or 0.205.

Now, this is rougher'n a corn-cob. Lotsa guessing. But with real, factual numbers for the ID, the strength of the steel and the area of the lugs, it's still the same sort of calculation.

FWIW, Art

 

[ClarkEMyers]

A second thought for case ID - seat a bullet just forward of the head.

A second thought gives a better value for case ID on the '06.

People have made variations on the .45 Super from '06 and .308 Win cases without too much wasp waist so the ID can be taken at something in the .451+ range. Shape ought to be pretty much irrelevent - balloon head having no larger effective area and inside taper no smaller or we could taper to a sharp point and load to astounding pressures.

Similarly the 222-223-5.56 range is sometimes used to make things like the 9mm Super Cooper so the 0.35 ID is likely enough smaller than the '06 to notice.

Many readers will have played with hydraulic cylinders and be familiar with the notion of pressure on the ram versus PSI of the hydraulic fluid.

An interesting reminder from the finite element analysis is that the stress is not evenly distributed. There might even be some leverage involved.

Interesting that modern machining practices have led to a more even distribution of the stresses and associated with this has been a reduction in observed pressure signs - see e.g. some of the writing by John Barnsness where he talks about the loads Layne Simpson worked up for the 7MM STW as a wildcat with easy bolt operation on loads in the 75,000 PSI range.

 

[SRMohawk]

Gentlemen,
Those of you who have elaborated on bolt thrust specifically (e.g. cracked butt, Drue, etc.) have made some pretty illuminating points. I thank you for that. Will have to carefully examine some of my handloading practices in light of all this new knowledge once I get this .300 WSM back from the smith. Also, for those of you who have suggested that the problem herein lies with my having pushed the envelope too far, I am increasingly suspicious that this practice is to blame. When last I shot my .300 WSM before turning it in for rebarreling, I chronographed 185-gr Lapua Scenars taking off at over 3100 fps. (This was NOT my intention, incidentally. Something had happened to the barrel that wasn't showing up in the bore-scope.) And after having made a call to Winchester/Olin, a gentleman there told me that even with a 28" barrel, this cartridge was NOT designed to push projectiles this heavy at such speeds. As for why I've not seen the tell-tale signs of excessive pressures in the brass, it turns out that these often just don't show up in tight, custom chamber jobs like they do in factory chambers.

As for those of you who think this is a matter of shoddy maintenance or ignorance of much more basic knowledge of factors that contribute to poor accuracy, go back and review the thread 'Short Barrel Life'. I assure you, I cover most all the bases when it comes to things like throat erosion or copper fouling. If I observe rapid or excessive throat erosion in a new barrel, for instance, I won't just adjust the OAL of my handloads and keep shooting. I'll take it up the road and have my smith bore-scope it to see if the throat is wearing evenly or if it's getting rough, as well. And maintenance . . . hell, it should be a foregone conclusion among those of us who even spend the $$$ on custom rifles (around $4000/rifle before optics) that if a new barrel starts to copper (withstanding religious adherence to prescribed break-in/cleaning procedures) that the barrel comes off and goes back to the manufacturer!

Zak,
The .338-caliber barrel that went south on me several years back was a Hart barrel. Historically, the load I've shot more of than anything incorporates Lapua brass, a Lapua 250-gr Scenar, 89.5 grains of VV N560, and a Federal 215M primer. I was getting awesome results with this recipe, too. I'm talkin' sub-0.5 MOA all the way out to 1 km (off a bench, of course)! A competitive shooting acquaintance of mine once shot a group that measured a little under 7" 1050 yards out off a Harris bipod with this combo! Then the bottom fell out after the 650 round mark and the rifle wouldn't shoot under 10" at 600 yards. This, incidentally, is why I was asking you about propellants at that long range optics thread.

Okay, fellas. That's all from here. I'll keep you all updated on these projects.

SRM