Blackpowder pressures, calibers, steel and wall thickness

[Powderandball]

Does the ”honeycomb” construction of a revolver cylinder make it stronger than a single barrel? The reason why I’m ask is because I’m trying to figure out the minimum safe wall thickness for a single shot black powder pistol. The pistol I’m making will be loaded with a .53 roundball (weight 231 grains) and the maximum charge I’ll ever use behind that roundball is 80 grains of pyrodex P (FFFG). In which pressure range will such a load be within? It’s hard to find pressure values on black powders loads, however it seem to me that black powder is in most cases low pressure compared to smokeless. In comparison, will the load I’m using have higher or lower pressure compared to the pressure generated by a 28 gauge modern shotgun?

According to this resource https://www.cip-bobp.org/homologation/uploads/ciptexts/a-4-1-en.pdf the minimum wall thickness at the chamber area for a 28 gauge shotgun is 1.05mm of best steel, and 1.9mm if you use the weakest steel that’s included in the list. With black powder, where does the pressure peak? In a black powder revolver, does the pressure peak in the cylinder or in the barrel?

 

[billraby]

Depends on the type of steel. Is it forged, cast, hot rolled, cold rolled, etc. Was it heat treated? If so, how? I really hope that you are not just trying to save a couple dollars by using a pipe that you have laying around for a barrel.

 

[Rifleman1776]

For pistol use that charge seems excessive. Not likely the short barrel will consume it all. Meaning performance won't be much different than a lesser charge. (I now hear footsteps running for the Lyman bp manual. ;-) Can't answer about the 'peak'.

 

[Powderandball]

Depends on the type of steel. Is it forged, cast, hot rolled, cold rolled, etc. Was it heat treated? If so, how? I really hope that you are not just trying to save a couple dollars by using a pipe that you have laying around for a barrel.

I’m searching for a proper steel to use for this project. I will drill the barrel from a barstock. I’ve been looking at a steel named 34CrNiMo6+QT, I can easily get my hands on this steel and it’s already quenched and tempered. According to the charts of this material it seem to come as both cold drawn and hot rolled barstock, however the mechanical properties are similar.

Regarding the chart of minimum wall thickness in the source above, the steel seem to be within the range of grade 3, however there’s a range of mechanical values that makes it difficult to categorize. Here’s the mechanical properties of my steel:

Tensile strength, Rm: 900 – 1100 MPa.
Yield strength, Rp0,2: min. 700 MPa.
Elongation, A5: min. 10-12%
Hardness: HB 275 – 335

700 is the minimum elastic limit for category 3, and the combination of hardness might be bad?

The minimum wall thickness with class 1 steel for a 20 gauge or smaller shotgun according to the chart is 1.9mm and for category 3 1.5mm. If I go for 2mm wall thickness with 34CrNiMo6+QT, will that give me enough safety factor for my black powder pistol?

 

[Powderandball]

For pistol use that charge seems excessive. Not likely the short barrel will consume it all. Meaning performance won't be much different than a lesser charge. (I now hear footsteps running for the Lyman bp manual. ;-) Can't answer about the 'peak'.

The charge I’ll actually use for this pistol will be 55-65 grains, however I want a safety factor so I want it to be able to handle excessive loads too, up to 80 grains.

 

[hawkeye2]

Are you planning to make your barrel from scratch? If you buy a commercial barrel then the wall thickness issue will be solved for you. No one will manufacture a barrel that isn't safe.

If you are attempting to arrive at a barrel wall thickness on your own you need to take some things into consideration. First, the steel used in barrels for smokeless powder is far stronger than what is commonly used for black powder barrels. Second, you need a good understanding of internal ballistics. Winchester did extensive research in the late 1950s comparing the pressure curves between smokeless and blackpowders in shotguns including graphing pressures for each inch of the barrel. You should be able to find that info by searching. Third, you will need a significant safety factor built into the barrel, probably a minimum of 50%, more would be better. Fourth, other things need to be considered, for example the sharp corner at the bottom of each groove in a rifled barrel can act as a stress riser. Last, I suggest you ignore anything that happens in a revolver and the construction of same. It will only lead you down a gopher hole and waste your time. If you understand the aforementioned you will have only have the basics of barrel design. Knowing the properties (tensile strength, yield, etc.) of the steel alloy you are considering there are ways (more than one) of calculating the wall thickness. OBTW, give thought to the machineability of the chosen steel too.

 

[FishDFly]

"In a black powder revolver, does the pressure peak in the cylinder or in the barrel?"

Peaks in the cylinder.

You are going to rifle the barrel yourself?

 

[Powderandball]

I’m not making a rifled barrel and I’m not making a shotgun, but a smoothbore muzzleloader, loaded with a patched roundball and pyrodex. I will make the barrel from scratch. I want the walls to be resonable thin, so my idea here is to use a high strength steel and load it with pyrodex/black powder only. If the wall thickness of a particular steel is strong enough for a known smokeless pressure that’s higher than the pressure generated by my pyrodex load, then that same wall thickness in my black powder pistol will give me a safety factor. That’s why I’m wounder if I can apply the thickness of revolver cylinders to a single barrel as reference.

With especially soft steels a thicker barrel can have other advantages beside just withstanding the pressure, a thicker barrel is easier to keep round and straight for accuracy and avoid dents. Perhaps that’s the reason why barrels are usually made thicker than revolver cylinders? I read that the wall thickness of a M1911 pistol barrel is 0.065 inches, a .45 ACP generate 21,000 PSI. I don’t know if the thickness was measured at the chamber area though.

The problem I have is that I don’t know for sure what’s the maximum pressure of my load, so I don’t know what I should compare with. According to this chart Calculating Barrel Pressure and Projectile Velocity in Gun Systems | Close Focus Research - Ballistic Testing Services a 28 gauge shotgun generate max 12,500 PSI.

 

[Zonie]

A cylinder used on a revolver has a capacity that will only hold a limited amount of powder so I wouldn't use the wall thickness on those as a guide for the barrel thickness on a single shot muzzleloading pistol.
Also, IMO, 1.9mm is only .0748 inches thick. That is much too thin for a barrel wall on a single shot muzzleloading pistol.

 

[Nameless Hunter]

As a starting point, what is the barrel thickness for a new smoothbore rifle and pistol that is available for purchase today?

 

[JCKelly]

As a starting point you might check exactly what your personal insurance policy covers.
This whole thing sounds like a bad idea. Fun, of course, but there might be more to it than you read on muzzle loading sites.
Buy a gun from some Spanish or Italian maker that has a good idea what they are doing. Modify it to suit your idea of style'

Well, what a dumb thing to say! You are sure a-gonna make your own!

Steel. At least do not use any free machining steel. Sometimes called "resulfurized" or "leaded". Some have phosphorus added. Never, ever use a steel called "12L14L Yeah, a lot of people do but I have an unpleasant list of shooters whose bodies have been modified by using barrels made of 12L14 steel.
Strength is nice but toughness and ductility are more important when something goes wrong. Best choice is to have something called "Rifle Quality Steel" or, I think, "Gun-barrel Quality Steel" Chances are about non-existant of finding such outside of a custom MODERN gun barrel maker's shop. In practice best you might do is get some annealed 4130 or 4140 bar (not ever tube or pipe) and drill it out. 8620 may also be OK
I made my first pistol I think in 1959. Used a modern .45 ACP barrel with my own half-a---- breechplug. Didn't shoot it much. But it was fun. First rifle made couple years later, used a modern .44 cal blank. Happily, modern made muzzle loading barrels were not yet available.
Studied metallurgy in collage, worked my whole career as a metallurgist. Heat-treated steel, stainless steel and finally special high temperature alloys were my thing. More to the point here, I was also an expert witness for the plaintiff in several muzzle loading barrel failures.

 

[Powderandball]

As a starting point you might check exactly what your personal insurance policy covers.
This whole thing sounds like a bad idea. Fun, of course, but there might be more to it than you read on muzzle loading sites.
Buy a gun from some Spanish or Italian maker that has a good idea what they are doing. Modify it to suit your idea of style'

Well, what a dumb thing to say! You are sure a-gonna make your own!

Steel. At least do not use any free machining steel. Sometimes called "resulfurized" or "leaded". Some have phosphorus added. Never, ever use a steel called "12L14L Yeah, a lot of people do but I have an unpleasant list of shooters whose bodies have been modified by using barrels made of 12L14 steel.
Strength is nice but toughness and ductility are more important when something goes wrong. Best choice is to have something called "Rifle Quality Steel" or, I think, "Gun-barrel Quality Steel" Chances are about non-existant of finding such outside of a custom MODERN gun barrel maker's shop. In practice best you might do is get some annealed 4130 or 4140 bar (not ever tube or pipe) and drill it out. 8620 may also be OK
I made my first pistol I think in 1959. Used a modern .45 ACP barrel with my own half-a---- breechplug. Didn't shoot it much. But it was fun. First rifle made couple years later, used a modern .44 cal blank. Happily, modern made muzzle loading barrels were not yet available.
Studied metallurgy in collage, worked my whole career as a metallurgist. Heat-treated steel, stainless steel and finally special high temperature alloys were my thing. More to the point here, I was also an expert witness for the plaintiff in several muzzle loading barrel failures.

Do you have any experience with 34CrNiMo6 in quenched and tempered condition? I’ve read multiple steel charts regarding mechanical properties such as high tensile strength etc. and it seem to me that this steel is a good option. For example 4140 has 655 Mpa in tensile strength and 414 Mpa in yield strength in annealed condition.

34CrNiMo6 (in quenched and tempered conidition which is what I’ll use anyway) has 900-1100 MPa tensile strength and minimum 700 MPa yield strength. However, in quenched and tempered condition 4140 has 1075 Mpa tensile strength and 986 Mpa yield strength according to this site SAE AISI 4140 Steel Properties, Material Heat Treatment, Rockwell Hardness - The World Material

According to this site 8620 steel doesn’t exist in +QT condition and has much lower values AISI SAE 8620 Steel Properties, Chemical Composition, Equivalent, Heat Treatment - The World Material

In +QT condition 4130 has 1040 Mpa tensile strength and 979 Mpa yield strength. 34CrNiMo6 seem to be harder than the steels you mentioned though, with a HB value of 275-335.

Btw I think I’ll go down to caliber .50 which will reduce the weight of the ball to 177 grains and I’ll also reduce the maximum charge to 70 grains.

Regarding the chemical composition. In the resource for minimum wall thickness and steel that was linked to earlier, it’s stated that: ”For firearms, certain of the alloy ingredients call for particular attention: The maximum content of the elements phosphorus and sulphur must be 0,025 - 0,035%. A high degree of steel purity sought after from the point of view of elasticity is linked with some loss of machineability. In this connection, ISO Standards provide in each case a maximum and minimum percentage for sulphur content.”

When I look at the steel you mentioned and compare with 34CrNiMo6 I don’t see that there’s a different here, atleast not that disfavour 34CrNiMo6. According to this site https://www.steel-grades.com/Steel-Grades/Special-Alloy/4140.html 4140 have P= 0.04% and S= 0.045%.

For 34CrNiMo6: P= 0,015% and S= 0,030% as typical composition. The maximum value I found was P=0,035. It was harder to find a max value on S but it seem to be within the range 0,020-0,035 (how does higher or lower sulfur content affect a gun barrel?)

However, it’s possible that the 4140 resource on this is too versatile and that the specific kind of 4140 that’s used in barrels has lower values.