Bolt Thrust Comparisons

[COSteve]

Deleted

[Thunder50]

Would you use case diameter or rim diameter, since you would have greater surface area against the bolt with rim dia.?

[adirondakjack]

Would you use case diameter or rim diameter, since you would have greater surface area against the bolt with rim dia.?

The entire case surface would be equal in theory, assuming a dead flat bolt face and no headspace. Now, here's a wrinkle. We know that during firing, primers back out somewhat as pressures rise and are re-seated as the case slams back over the primer (how much motion we get depends on "cammed back" headspace (when the bolt is loaded rearward). It would seem the energy needed to slam tha case back, including overcoming any existing case wall-to-chamber drag (case is expanded at that point), would be factors mitigating ACTUAL rearward bolt thrust. Some of the energy would also be absorbed by bowing the case head a bit momentarily until it gets ironed back flat against the bolt face. In other words, those factors would help limit the increase in bolt thrust as pressures increase, so thrust to case pressure might NOT be a linear relationship......

What got me thinking of this is .22 Hornet in a revolver. One of the ways they limit the potential for tying up the gun as brass slips back and the case shoulder getrs re-formed is by keeping chambers dead dry and slighly rough to increase the case "grab" in the chamber and limit "bolt thrust" enough so that as the brass rebounds from expansion and stretching, it leaves enough clearance behind the case to allow the cylinder to turn. Early iterations were so bad, before these techniques were developed, ya had to pound the cylinder of a DA revolver to get it to turn, then drift the brass out.


Further pondering leads me to believe the whole formula is whacked because NO, in a real world situation, where you have ANY headspace, the case will grab the chamber, the case head will actually have space behind it initially, then the case, while still grabbing the sides of the chamber, will BOW at the case head, in a shape resembling a parabolic dish, both because of the edges being attached to the sidewalls of the case, and because the case head is thinnest in the center, so any round fired at enough pressure to seal the chamber (cases grab the chamber walls) will have different forces applied to the case head, little at the edges (if any), and more in the middle.

This is precisely why magnum rifles with excess headspace suffer case head separation.

At very high levels the case will be pulled free of the chamber walls and slammed back with great force, but only after it has been bowed, so even there, the impact of case head to bolt face is a much more complex and dynamic, almost shock-absorbing process that the formula is insufficient. For say 15K or less, sure, it works, but much beyond that, I fear it falls apart. For a middling load at say ACP pressures, ya likely have a case head slamming back over the priner with residual energy AFTER pressures have reduced enough to allow the case to let go of the chamber walls......

[COSteve]

Would you use case diameter or rim diameter, since you would have greater surface area against the bolt with rim dia.?

The formula actually calls for the inside case dia, not the outside or rim dia. The inside diameter wasn't available so I used the base dia as provided by Wikidedia for each caliber.

adirondakjack's discussion has some interesting points, however, because the brass case is pliable, the bolt is going to see the thrust. To be sure, this chart's usefulness to me is more of a comparative nature than an absolute one. By that I mean that looking at the calibers, I compared the calculated bolt thrusts and decided that the 14,000psi 45 Colt and the 22,900psi 38 spl produce comparable levels.

As the Uberti '73s have lower bolt thrust tolerance than, say a '92 clone, I was looking to evaluate the suitability of my hot 38spl loads vs using some 45 Colt +P loads on a regular basis. Clearly, the 45 Colt +Ps generate a significantly higher bolt thrust than even the .357 mags and both Nate and Paco don't recommend even a .357 mag level as a steady diet in the Ubertis.

As I said above, because that is my intent, I'm now looking at only the .357 mag version.

[AJMD429]

Clearly, the 45 Colt +Ps generate a significantly higher bolt thrust than even the .357 mags and both Nate and Paco don't recommend even a .357 mag level as a steady diet in the Ubertis.

Do they make the Ubertis in .44 Magnum? If so, you'd think they'd not last as long as the .45 Colts fired with "+P" loads, given the thrust differences.

BTW - It's all your fault but now having looked at that chart, I'm gonna be NERVOUS the next time I fire my .454 Casull...

[adirondakjack]

Uberti has a prototype 73 in .44 manglem, but I have my doubts they will survive the rigors of the Italian proof house without developing noticable slop.

[KirkD]

I have an article by P.O. Ackley where he experimented with screwing out the barrel of a 30-30 and shot factory loads in it. What he found was that there is zero bolt thrust. The base of the cartridge did not come into contact with the bolt face, except if the cartridge was lubricated with oil. Upon firing, the brass of the case sealed against the chamber walls, preventing the case from moving backwards. This was for a 30-30. I imagine that other cartridges may vary. I know that if I have soot on the outside of the case, I assume that the bolt face is taking the thrust. I like to find a load that will seal the case walls against the chamber walls, preventing soot on the outside of the case and preventing thrust against the bolt.

[CJM]

And then there is the theory that you don't have to worry about bolt thrust at all (unless you oil your ammo), but that lever guns will always blow out the bottom of the chamber due to the lack of frame support between the barrel and the magazine tube.

[J Miller]

Perhaps I'm a bit of a simpleton but things like bolt thrust are for engineers and designers.
The 1873 action was designed a long time ago for very low pressure cartridges. However it's been updated with modern metallurgy and manufacturing methods, and today's version is probably a lot stronger than we give it credit for.

If Uberti chambered this gun for a given cartridge it had to pass their tests. I'm sure they were not filling 357 cases with black powder either.
These guns all have to pass the Italian proof specs and these include the European CIP pressure tests.
At one time I had a chart giving the side by side comparison of the CIP specs and SAAMI specs. In almost every instance CIP specs were hotter than SAAMI.

So, rather than over think the equation and give myself a headache I'd keep the loads within SAAMI limits for the .357 round and enjoy shooting my rifle without so much as a second thought.

Joe

[COSteve]

Actually, I talked to Nate about Uberti's 44 mag '73. Uberti has yet to ship the first one so at this point it's 'vaporware'. Uberti did reply to that their 44 mag will only be available in blued receivers because the steel in them will be higher strength.

As to the question of whether bolt thrust is in fact real, and not criticizing any posters here, anyone who thinks that a soft, thin brass case can contain 15,000 psi or higher is ignoring the physical limitations of the metal. In truth, the thought that a piece of brass could withstand that pressure level is just plain silly. Why do you think cases stretch? Why, if a case bursts, does it always burst about 1/4" up from the base?

The facts are that a steady diet of hot loads in '73 Ubertis increase the headspacing to a point where they can become dangerous to shoot. Why are they dangerous? Because of explosive case ruptures that can cause damage to the weapon and/or injury to the shooter.

Think about it; if a brass case can contain the charge's pressure, why do we care about excessive headspace at all? Ask our own Nate here if that isn't what he sees. Ask any gunsmith what is shearing off bolt lugs on hot loads.

Something is doing it and that something is called 'bolt thrust'. Something is causing the primers to be pushed back into the case and that something is called 'bolt thrust'.

[williamranks]

I got a little scared until I realized the chart said inch pounds.

[COSteve]

I got a little scared until I realized the chart said inch pounds.

Time to get scared. The numbers are in lbs per square inch, not inch pounds per square inch.

By the way, I saw Gunsmith Talk's article about the 30-30 case and citing P.O. Ackley's discussion but his (Gunsmith Talk's) conclusion makes no sense as there are many, many discussions of excessive chamber pressure overloading the bolt lugs and causing them to bend or shear off. If the brass case contains all of the pressure and the case locks to the chamber walls, there wouldn't be any load on the bolt lugs at all so how or why would they shear with excessive chamber pressure?

The fact is that the bolt lugs 'see' and contain the chamber pressure and that's why they can be damaged by overpressure events. For those who'd like to read more about this, Wikipedia has a good writeup HERE.

[adirondakjack]

I never said bolt thrust didn't exist, as I know it does. I said because there is headspace, and because an expanded case grabs the chamber, the FORMULA for bolt thrust actually absorbed by the bolt itself is much more complex than the one shown. I would call this a "gross case head thrust" formula.

Once again, at very low pressures where the case does not seal, the bolt thrust seen is a function of pressure, period. The case tries, under pressure, to squirt out the back like a blowgun dart.

At middling pressures, the brass case grabs the chamber, mitigating the rearward thrust of the case, and some of that energy is absorbed by the case, at least until pressures recede to a level where the case can move.

The higher the pressure, the more the case wants to grab the chamber, while at the same time the pressure stretches the case head backward into the bolt, and the more important close headspace becomes, lest the case metal instantly be overwhelmed and the head separate from the sides.

In every instance, reagrdless of initial pressure, whatever pressure NOT absorbed or mitigated by the rebounding qualities of case brass is in fact taken by the bolt and transmitted to the lugs. Paco's articles where he loaded hot loads and measured bolt lug peening in fairly short order in even the strong '92 action is instructive.

The bolt simply doesn't take ALL the pressure, and a much more complex model would be needed to give us more accurate data. Some day, somebody will insert a piezeo wafer on a bolt face and measure it....

[AJMD429]

Uberti did reply to that their 44 mag will only be available in blued receivers because the steel in them will be higher strength.

Wow. I didn't think stainless was necessarily less strong vs. blued...

I'd always thought my stainless Guide Gun with its thick-looking round stainless barrel was maybe just a bit stronger than my blued 1895 Cowboy with its delicate-looking octagonal blued barrel.

Too bad the levergun manufacturers didn't build in just a bit more 'lift' to the carrier systems, so there could be more thickness of metal between the barrel-hole and magazine-tube-hole.

[COSteve]

I don't think that was what they were referring to but rather that you won't be able to get it in the 'case hardened' look. I don't know enough about in-depth metallurgy to comment on the carbon steel vs stainless steel differences.

[adirondakjack]

I don't think that was what they were referring to but rather that you won't be able to get it in the 'case hardened' look. I don't know enough about in-depth metallurgy to comment on the carbon steel vs stainless steel differences.

Exactly. SS would be fine, but case hardening soft steel not so much. Using ordinary bluing, they can use steel, maybe forged, so it will be tough all the way through. As to blue VS SS, they probably figure on starting with a single option, see how that goes. So many of these rifles are used by historical buffs, CAS types, etc that .44 manglem may not be that big a seller.

[Ben_Rumson]

It will be interesting to see what the historical buffs have to say about the 44mag as they are willing to overlook 45 Colt chambered 73s and 92 types...

[KirkD]

I have a copy of P.O. Ackley's article complete with photographs of the various cases shot under the circumstances he experimented with. He did do the experiment and he did find that factory 30-30 ammo could be fired without making contact with a bolt, provided the case was dry and unlubricated. I cannot recall if he mentioned it or not, but I am sure the cases would stretch if there was no bolt to arrest the stretching. Thinking about this, and reflecting upon my own experiences with cases of different cartridges and various headspaces, here are some inferences I will make:

• 1. For some cartridges, zero contact with the bolt face will allow stretching of the case, but without case failure, at least on the first shot with a brand new case.
  2. For other cartridges, the pressure vs. axial strength of the case walls may well exceed the capabilities of the case, and the case will fail on the first shot if there is no bolt in place to arrest the stretching.
  3. Stretching of the case will occur in the rear portion that is too thick to seal against the case wall. That is why we get case head separation just forward of the rim about 1/8" to 1/4".
  4. For guns firing cartridges described by (1) and having excessive headspace, there may be zero bolt thrust on the first shot with a brand new case. Subsequent reloadings may stretch the case to where it begins to make contact with the bolt face and begins to exert some bolt thrust upon firing.
  5. For guns for which the bolt tightly contacts the case head, there will be bolt thrust equal to the force necessary to stretch the case if there were no bolt contact. In other words, the less the bolt permits the case to stretch, the greater the bolt thrust.
  6. I have not looked at the formula to compute bolt thrust, but it should be mitigated by how well the case seals against the chamber walls. In other words, a case that does not seal at all against the chamber walls (i.e., covered in soot) will exert more force against the bolt than a case that seals well.
  7. A cartridge can be so hot that it will damage the lugs. At that point, any sealing against the chamber walls is insignificant in comparison to the excessive force generated by the hot load.
  8. I suspect that the greatest damage to the action is sustained when the headspace is slightly excessive, allowing a high pressure cartridge to slam back against the bolt, creating an impact.

[adirondakjack]

Kirk, I have an 8MM mauser that has a bit generous headspace. I found this out because fired cases came back with high primers, yet the shoulder of the round didn't get pushed back.......... Your theory (and mine) in action.

[KirkD]

Looking at Steve's question as to what the bolt thrust would be for a 45 Colt, I think the actual bolt thrust would be reduced by the strength of the brass forward of the rim, where it is thick enough not to temporarily weld to the chamber walls. I don't know what the wall thickness at that point would be, but let us say it is .015". Using Steve's formula, the axial thrust generated would be ...

14,000 x pi x ((.480-.030)/2)^2 = 2,226 pounds

The force absorbed by the brass walls, without undue stretching (yield strength) would be approximately ...

2*pi*(.470/2) * .03 * 29,000 psi = 642 pounds

So actual bolt thrust may be closer to 1,584, depending upon how tight that bolt is against the rim and depending upon how thick the case walls are in that portion of the web that fails to temporarily weld against the chamber walls. I suggested .015" but it may be twice that, significantly reducing maximum bolt thrust.

Using ultimate tensile strength of brass would allow it to absorb 1,765 pounds of thrust if the web wall thickness is .015, although the brass would be unsafe to use anymore. We aren't interested in that.

All these numbers are approximations for the 45 Colt.

[adirondakjack]

Lets do the math for .45 ACP while we're at it. We routinely reload ACP cases after use in pistols without full case head support. FWIW the case thickness of .45 Colt ahead of the taper is .012 or so, but by the time we got to within say 3/8" from the inside bottom, it is quite a bit thicker, maybe on the order of .020+ before turing the corner at the bottom. The case bottom web is pretty stout, even at the primer pocket, and the part beyond the primer is very thick....

[Hobie]

You guys know Ackley had a protege, one Mike Bellm, who is heavy into Contenders and Encores. In fact that has become a niche he's filled in the gunsmithing world. The tip up systems are subject to stresses which eventually can cause them to be off the face or, IOW, have excessive headspace as either the bearing surfaces or frames wear and/or stretch. The original Contenders (as distinguished from the G2) are almost notorious for frame stretching when cartridges exceed the breech thrust parameters acceptable in this action. Thompson Center replaced many frames (under their lifetime warranty) due to shooters loading the cartridges such as the .35 Rem (and many wildcats) to exceed the acceptable limits.

Calculations of pressure as it bears against the breech should use the internal surface area of the cartridge case but most shooters use the external case head diameter in the calculation as it is easier for any shooter to find that info with a pair of calipers. The upshot of the calculation is that larger diameter cases need to run at lower average pressure than the smaller cases. Thus the .357 Max and .223 Rem can run at 50K PSI but the .45-70 can't exceed 28K PSI. When the bigger diameter short mags and super short mags came out some Encore barrels were so chambered without this consideration because the system had been safely using some rather big rounds. However, the Encore isn't capable of handling those cartridges. Why? Breech thrust. At operating pressures those cartridges produce breech thrust that is excessive for the Encore.

[Old Time Hunter]

Why do you think cases stretch?

Absolutely! If this did not happen, why would you need a bolt at all? There is no way the brass can expand and retain the equal opposite force without the bolt. You would have to beat the spent cartridge out every time you shot it. How much resistance the circumference of the shell has against the chamber upon expansion is another question all together cause that requires not only the total surface area of the shell, but the physical characteristics also. Theoretically, a modified bottle neck shell like a .44-40 would have less surface resistance than a .44 mag even though they are of similar length.

[pokey]

wow, math, applied to physics, directly impacting [:D] internal ballistics. lovely.
you math magicians keep it up.

too many unknown[unknowable?] variables.
maybe the OP just wanted a comparison to choose by, not perfectly exact calcs ?

[COSteve]

maybe the OP just wanted a comparison to choose by, not perfectly exact calcs ?

Exactly! And I said that in my second post, "To be sure, this chart's usefulness to me is more of a comparative nature than an absolute one. By that I mean that looking at the calibers, I compared the calculated bolt thrusts and decided that the 14,000psi 45 Colt and the 22,900psi 38 spl produce comparable levels." You see, my purpose was to quantify the relative bolt thrust between calibers because of my interest in '73 clone with it's inherently weaker action design than a '92 clone which I already own.

And, after relooking at the available calibers from Uberti / Cimarron, I find that the Special/Deluxe Sporting Rifles actually see similar bolt thrust pressures (under 2,600psi) from 3 calibers of interest to me; 1st, my current ≈ 23,000psi hot loaded 38spl, a SAAMI spec 14,000psi loaded 45 Colt, and a SAAMI spec 15,500psi loaded 44spl.

If I elect to go with another .357mag version, my hot 38spl loads are reasonable in it. If, however, I consider a new caliber, the 45 Colt has appeal but the much more common 44 spl is about the same diameter a '73's original caliber (the antique 44-40) all be it a bit shorter in OAL, is easier to handload, and generates about the same performance with modern smokeless powder. It might make an interesting round for the '73 as it will keep the pressure down to something more reasonable for a toggle link actioned levergun.

[BAGTIC]

I have always calculated bolt thrust based on chamber diameter. I think it unwise to depend on the case restraining some of the pressure as cases sometime fail and sometimes they fail to adhere to the chamber. This is a useful safety factor.

If there is a head separation the working area of the gas then becomes the entire diameter of the chamber as the gas flows into the gap between the two separated halves. Better safe than sorry.

Yup! I wear a belt and suspenders.

[AJMD429]

I agree with COSteve's point of the data as a guide for relative pressures and stresses to expect on actions.

It is true that there are lots of 'brass' factors, and obviously many guns have 'chambers' that even when the bolt is in battery, wouldn't hold water (or for that matter, molasses) for more than a second or two before it would leak out. I realize that during that 'second or two', a gun can become a grenade, but my point is that unless we're shooting a muzzle-loader, we always assume there is a plasticity and 'sealing' effect on the part of the brass case. If a very thin area of brass isn't supported for even a small gap, it will rupture, yet the thicker case-head and 'web' areas clearly hold tens of thousands of pounds of pressure per square inch, over boltface/chamber gaps measured in hundredths or even tenths of an inch. Perhaps the good news is that the guns with the most 'unsupported' area, like an overly-ramped 1911, operate at lower pressures, and are designed so that failure of the case will likely result only in a bulged case vs. a ruptured one, and even a ruptured one will often be far less than catastrophic. Some semiautos don't even 'lock' their cartridges in the chamber, and just use a spring to keep them in battery! On the other hand, our super-high-pressure rounds we might fire in a bolt-action gun with lots of locking-lug surface area, usually have extremely little 'unsupported' area, and are designed so that in the unlikely event that the brass fails to seal, the chamber will stay closed, and the gas is directed in a relatively safe direction.

So, we always depend on the brass to seal the chamber to some degree, but the action design has to set that 'degree' to within safe limits which vary greatly depending on the pressure of the cartridge. COSteve is merely using that 'psi' data to see what other cartridges might be acceptable in a given design, based on the others it has 'proven' itself capable of.