Bolt thrust

[JMayo]

. Kinda of common sense the thinner spots go first with the lugs tearing out at the edges.
Still a pressure situation reliving where it can.

 

[dellet]

Bolt thrust failure has little or nothing to do radial pressure or a single over charged cartridge or maybe even a couple dozen, unless it’s a poor design. Consider proof loads are 125% of max loads.

A failure do to excessive bolt thrust is due to repeated blows to whatever keeps the breech from opening. In a bolt action, it’s the locking lugs. In a rolling block it’s the hammer.

The way you would test shear strength of a Remington 700 bolt action would be to make a jig with a rod the size of the inside of whatever case head you prefer replacing the barrel, set it up in a powered hammer and count the strokes it takes to shear the lugs or drive them through the receiver.

Or maybe get an engineer to calculate it base on materials and pressures.

Bolt thrust is a bit of black magic. The reason the primers in a 30-30 stick out after firing is because you have zero bolt thrust from the case head. You basically have more thrust from a 22 RF than a 30-30. Simply because the 22 won’t expand and stick the case to the chamber walls. It hammers back on the breach face. The only thing hitting from the 30-30 is the primer. There is enough pressure to stick to the chamber, not enough to stretch the brass and reseat the primer.

Same reason a case lubricated in fire forming expands forward at the shoulder, instead of stretching at the case head where it is largely unsupported and not sticking to the chamber. The lubricated case, slides back to the breech face and expands forward. A full pressure load sticks the case to the chamber and the expansion happens at the least supported area of the case.

If you doubt the holding power of an expanded case in the chamber, P O Ackley did a reasonably famous test by unscrewing a barrel and firing until failure occurred. Or talk to guys shooting over gassed AR style rifles in high pressure cartridges that pull the case heads off.

So what most people see, is a rifle that had enough pressure to first separate the case head and drive it back into the breech face at full pressure.

To get a better idea of what excessive bolt thrust can do to a rifle, look at lever actions, rolling blocks, falling blocks and such. Rifles where they have a history of “shooting loose”. Hard to imagine a rifle design that all the force of the cartridge was held back by a 1/8” screw the hammer pivots on, but they are out there, and when that screw bends, the actions no longer stay closed. Many have probably shot a 22 that blows out at the rim. Headspace created from bolt thrust.

There is a reason bolt actions evolved. Some of the early ones, straight pulls are a good example, really weren’t any stronger than something like a Sharps Borchardt. Probably actually weaker.

 

[K22]

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.

Your probably correct but those conditions posted are the ones most cited in numerous publications regarding warning signs of high pressure. The least reliable in my opinion is the appearance of the primer. But hard bolt lift and extractor marks are sure signs of high pressure in my experience.

For example, I had a pressure surge several years ago with H380 in a 22 250 in a 90+ day in the field hunting ground hogs as evidenced by hard bolt lift and clear extractor deformation mark on the rim of the case. This load had been tested and was fine at the range under shaded and cooler conditions.

I've seen cases with bulges in the head area fired from bolt rifles, the bulges not due to high pressure but to lack of concentricity of the rifle chamber as verified by a rifles smith. Maybe this is not the area you are referring to. Sierra published a paper on this issue, and I think it was posted in the daily bulletin some time ago. Would like your thoughts of this. Thanks.

 

[rijndael]

This was a nugget I copied from Hodgdon's site a while back.

https://www.hodgdonreloading.com/simple-trick-monitoring-pressure-your-rifle-reloads

Lower pressure rounds, like the .30-30 Winchester, usually yield maximum pressures at .0003"-.0004" expansion. Modern cartridges, like the .223 Remington, will show maximum pressure at .0004"-.0005", while .308 Winchester, .270 Winchester, etc., typically yield .0005"-.0006" expansion at max pressure. Magnums, like the .300 Winchester Magnum, show maximums at .0006”-.0007” expansion, and should be measured on the belt.

 

[JSH]

Throwing AR platform into the ring, dunno what kind of thrust you would call it. 300BOO mistakenly loaded into a 5.56 AR, some kind of high end bull pup, uh shall we just say that it was interesting. The kid walked away from it with out leaking any red fluid. One reason you will NEVER see one in my stable. Played with them years ago when JD Jones was playing with the whispers, if they had kept the 190-220 bullet as original that solved the chambering issue.
We have all read about it, holding the stuff in your hands, thinking about, impressive while nerve wracking at the same time.
I need to get to the shop and get pics if it’s still there.

 

[powderbrake]

Not to change the subject, but as long as my loads are not excessive, and I have correct lubrication in my action, there is nothing I can do to control thrust on my lugs. I have to trust in the engineering of the action. Short range benchrest 6PPC shooters overpressure the heck out of their loads, and the actions last for many years.

So, I don't worry about bolt thrust. What I do worry about is cleaning and lubing the bolt and receiver lugs after each use of the rifle.
I use a Sinclair lug cleaner, and I grease the bolt and lugs with TW25. After 5000 to 8000 rounds on my varmint and benchrest rifles, the lugs are still smooth and slick..

 

[K22]

Not to change the subject, but as long as my loads are not excessive, and I have correct lubrication in my action, there is nothing I can do to control thrust on my lugs. I have to trust in the engineering of the action. Short range benchrest 6PPC shooters overpressure the heck out of their loads, and the actions last for many years.

So, I don't worry about bolt thrust. What I do worry about is cleaning and lubing the bolt and receiver lugs after each use of the rifle.
I use a Sinclair lug cleaner, and I grease the bolt and lugs with TW25. After 5000 to 8000 rounds on my varmint and benchrest rifles, the lugs are still smooth and slick..

I use the Dewey one but same principle. Those tools do a really good job of removing junk from the lug recess areas.

Amazing, something that actually works as advertised - rare these days!

 

[Steve Blair]

you might be correct, considering the lugs and their abutments are the thickest linear portions of the action.
But you can sure get enough in to split the receiver. There are numerous examples of this.

Explosions generally follow the path of least resistance.

You are correct, Jackie. The tenon/receiver hoop strength is generally much less than the lug/abutment shear strength. Small diameter cases like .223 would most likely fail the brass in an unsupported area. A magnum case more likely to rupture the barrel if the pressure rises fast enough.

 

[C_Ackley]

This was a nugget I copied from Hodgdon's site a while back.

I subscribe to Denton Bramwell's theory that measure case head expansion doesn't mean a whole bunch. He's got a good article on it.

https://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf

 

[BoydAllen]

In my - unlearned - opinion, lube is not going to help overcome the radial force of cartridge expansion in the chamber to permit the entire cartridge to slide backwards on ignition.

If that was possible, all we have to do to avoid case head separation is lube the cases.

The closed bolt in the action forms a pressure vessel with the chamber. The weak point is the chamber wall.

I'm not sure what the use case could be where it becomes practical to worry about bolt thrust before chamber pressure.

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.

 

[jsn]

One of the magazines (Rifle or Handloader) had an article years ago with some form of testing film used to measure bolt thrust on the face of the bolt.

I don't remember much about it.

 

[gunsandgunsmithing]

One of the magazines (Rifle or Handloader) had an article years ago with some form of testing film used to measure bolt thrust on the face of the bolt.

I don't remember much about it.

Interesting.

 

[AccelR8]

The way you would test shear strength of a Remington 700 bolt action would be to make a jig with a rod the size of the inside of whatever case head you prefer replacing the barrel, set it up in a powered hammer and count the strokes it takes to shear the lugs or drive them through the receiver.

Maybe we could get that guy who has the hydraulic press channel on the u-toobs to test one with his 100 ton press!

The only time I ever thought about bolt thrust was years ago when I was putting barrels on H&R Handi Rifles. Yeah, they don't have a bolt but I would do some quick calculations to be sure the cartridge I was chambering for didn't exceed the breach face thrust of the factory chamberings. Didn't want to stretch a receiver or pop an action open.

 

[dellet]

Maybe we could get that guy who has the hydraulic press channel on the u-toobs to test one with his 100 ton press!

The only time I ever thought about bolt thrust was years ago when I was putting barrels on H&R Handi Rifles. Yeah, they don't have a bolt but I would do some quick calculations to be sure the cartridge I was chambering for didn't exceed the breach face thrust of the factory chamberings. Didn't want to stretch a receiver or pop an action open.

There are a lot of people over the years who learned more than they ever wanted to know about bolt thrust/breech face pressure converting break open shotguns to rifles. It looked the same!

The other clue is in loading manuals. When you see multiple loads that are “strong action only”. 45 Colt loads for Ruger only, multiple “contender only” loads. 45-70 loads for trapdoor, modern lever actions and Ruger #1. It’s not the chamber/barrel that bursts. Rarely on the first few rounds, it’s multiple firings over a period of time, and you hope you see the crack while cleaning before it’s comes apart while shooting.

Always something to learn about.

 

[dave@aDave]

I'm very happy with my FN Mauser 98 Large Ring receiver and the bolt design.

 

[jackieschmidt]

Looks like we're talking bolt actions so far.

An AR15 is a different story altogether. There are numerous examples of bolt failures in them. Ironically,max load data for them is adjusted by the same factor as bolt thrust increases. IOW, a larger case head ID generates more thrust than a smaller one, at the same pressures. i.e....Grendel or ARC vs 223. If you calculate the bolt thrust for both, they are pretty much the same...at max recommended pressures for each. If you try to run a Grendel at 223/556 chamber pressures, bolt thrust is considerably higher and is how a lot of A15 Grendel bolt failures happen, in the first place. People try to load a Grendel or ARC to 556 pressures without accounting for the additional bolt thrust of the larger cases. Works fine in a bolt gun, so it's not the cartridge, but the AR15 platform's limitations for handling that additional bolt thrust. It's also not the thinner rim around the bolt itself. It is bolt thrust.

Just my 2 cents.

Someone already posted a link above to Dan Lilja's site/article on bolt thrust and how to calculate it if you want to confirm or play with the numbers a bit.

Yeh, all of this stuff has been gone oner many times, Clear back when PO Ackley tested vintage military actions, if I remember when actions did (finally) fail, only a few were due to lugs shearing off the bolt body with the rest of the receiver wrecked.

http://dutchman.rebooty.com/temp/Ackley.pdf

 

[Laurie]

In my - unlearned - opinion, lube is not going to help overcome the radial force of cartridge expansion in the chamber to permit the entire cartridge to slide backwards on ignition.

If that was possible, all we have to do to avoid case head separation is lube the cases.

The closed bolt in the action forms a pressure vessel with the chamber. The weak point is the chamber wall.

I'm not sure what the use case could be where it becomes practical to worry about bolt thrust before chamber pressure.

An oiled case does slide backwards. This was the standard British Army method used in proof testing throughout the 0.303 Lee-Enfield period and carried on into the 60s at least with 7.62.

Proofed 7.62 rifles are marked as 20T Proof and 303 was a fair bit less, but I can't remember what, and was the measured (?) estimated (?) pressure on the bolt face. I've no idea how they measured that pressure.

As people have occasionally found to their cost with the older Lee-Enfields (Long Lees and the No1 rifle / SMLE), a wet 303 case is also lubricated to some extent and increases bolt thrust substantially. For target shooters, it changes the elevation of the POI quite dramatically in the stretchy SMLE action. I've always found this a very strange trait in what is recognised as one of the best battle rifles ever, and which saw widespread use in the often very wet tropics and Asia. Maybe the use of charger loading and the 10-round magazine allowed users to keep cartridges largely dry in military use. It was certainly a problem though in target shooting as both Service Rifle and the subsequent Target Rifle (7.62 days) had mandatory single loading. As a result, there was a very clever leather shooters box/case that was placed on its side on the firing point and the lid plus side flaps (when in upright carry mode) folded up and sideways creating a cover. Inside and now horizontal when in use was a wooden slat with holes drilled facing the user making it a cartridge holder and allowing each round to be picked out while sheltered and moved across the short distance to the rifle action with minimal exposure to the rain.

 

[gunsandgunsmithing]

Yeh, all of this stuff has been gone oner many times, Clear back when PO Ackley tested vintage military actions, if I remember when actions did (finally) fail, only a few were due to lugs shearing off the bolt body with the rest of the receiver wrecked.

http://dutchman.rebooty.com/temp/Ackley.pdf

Must be a misunderstanding somewhere. Pressure is pressure AT A SPECIFIC CASE ID... WHICH TOGETHER, DETERMINES BOLT THRUST. So, AI or not has little or nothing to do with it if they were loaded to the same PRESSURES. Look at the link posted to Lilja's site above, by another poster. Again, probably a misunderstanding and probably on my part, but case shape, length etc, has little to do with how bolt thrust is calculated.

I apologize if I'm misunderstanding your post, though and I'm happy to listen. Yes, the lugs have proven to be a strong aspect of most actions but do appear to be the weak link specifically in AR15's...not AR10's even.

 

[Ray123]

PO Ackley wrote a lot about actions and relative strength of actions...and did many experiments, blowing up barrels on purpose....unscrewing barrels a half turn fire tbe cartridge, etc experiments.
With the 94 Win 32 gr of Unique double charge 150 gr bullet...blew up the barrel action was intact except for the threads were enlarged, shooter was unharmed. Win 94 Dry chamber test remove the locking block fire the cartridge with only the finger lever in place several times ...cartridges appeared normal...he says in his book.
There are calculations on bolt thrust...
Some mentioned ARs have a bolt problem where bolt actions do not...maybe in its best form the 5.56 it's already rather thin and fragile...then go to the Grendel or 450 bushmaster it's ridiculously thin little ribs undercut for case head. So fragile a bolt that lower pressures are demanded to compensate...THEN...Ya Add a lot of lube, everywhere it seems ...so the chambers may well contain a bit of lube from all the other well lubed areas to keep it running, or a lot of lube and/or cleaning agents, depending on how you "clean" the rifle barrel...this may help contribute to the broken lug problem experienced by some, by adding to bolt thrust with their oily chanbers....along with thin spiny AR bolts, that the best steels go into making...artillery bolts? Weatherby mark 5 bolts? 788 bolts? All seem to work...what's the difference...maybe the lube.

 

[big5ifty]

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.

Would this scenario not fall under the same as that observed with the 30-30 ?

Not quite enough boom to get the case to expand against the chamber, less observable when the walls are dry ?