Deep Six
01-27-2015, 11:47 PM
After reading some of the threads lately about the strength of the new model .44 special flattops, I decided to do a little investigating myself. First off let me share a little bit about myself. I am a mechanical engineer with a background in pressure vessel stress analysis working in the commercial nuclear power industry. While I’m no gun designer (my dream job!!!), pressure vessel stress analysis has some application in firearms. In this post I will primarily compare the .44 spl Flattop with the full-size NMB in .45 Colt. Please note that the following is my personal opinion and is presented for the purposes of casual discussion about the hobby we all share. Please do not take this as a professional analysis of any firearm or recommendation as to the safety of any particular load. Always start with the published starting charge and work up from there using your best judgment.
To start, I measured various revolvers from my collection using a digital caliper:
-------------------------------------NM Flattop-----NMB Bisley-----NM Flattop-----Redhawk-----S&W 629-6
-------------------------------------.44 Spl---------.45 Colt--------.41 Mag--------.44 Mag-------.44 Mag
Cylinder Diameter:----------------1.675”-----------1.731”---------1.731”---------1.782”---------1.709”
Cylinder Length:-------------------1.610”-----------1.705”---------1.672”---------1.750”---------1.702”
Cyl Outer Wall Thickness:---------0.085”-----------0.076”---------0.099”---------0.117”---------0.078
Cyl Web (between
charge holes) Thickness:----------0.064”-----------0.064”---------0.111”---------0.085”---------0.085”
Forcing Cone OD:------------------0.622”-----------0.622”---------0.622”---------0.685”---------0.625”
Top Strap Thickness:--------------0.248”-----------0.250”---------0.247”---------0.270”---------0.217”
Top Strap Width:------------------0.703”-----------0.720”----------0.713”---------0.705”---------0.665”
Charge Hole Diameter:------------0.460”-----------0.482”---------0.440”----------0.460”--------0.460”
The pressure generated inside the cylinder when a round is fired produces tri-axial stress in the wall of the cylinder. While a gun designer would be concerned with all three stress components, hoop stress is the only relevant consideration for the purposes of comparing the two revolvers. Hoop stress is given by the equation S = (P)(D) / (2t), where S is the hoop stress, P is the internal pressure, D is the mean diameter, and t is the wall thickness.
Since the wall thickness varies, use the minimum value. For the .44 spl, t is 0.065” at the web and D is 0.460+0.065 = 0.525”. For the .45 Colt, t is 0.065” at the web and D is 0.482+0.065 = 0.547”. If we use the same pressure load in both revolvers, the hoop stress is:
For the .44 spl: S = (P)(0.525) / (0.130) = 4.0385(P)
For the .45 Colt: S = (P)(0.547) / (0.130) = 4.2077(P)
Comparing the two, we see that for equal pressure loads, the hoop stress generated in the 45 is 4.2077/4.0385 = 1.042 times the hoop stress generated in the 44 spl. While the cylinder bolt notches definitely cause a stress concentration, they can be assumed to affect both guns equally for this comparison.
The forcing cone diameters of the two guns are the same. The top strap is slightly smaller on the flattop with a cross sectional area of (0.248)x(0.703) = 0.1743 in2 compared with the full size NMB at (0.250)x(0.720) = 0.1800 in2. The tensile load induced in the top strap will be proportional to the square of the inside diameter of the cases. For an equal pressure charge, the tensile load in the top strap of the .44 spl will be (0.430^2)/(0.452^2) = 0.905 times that of the .45 Colt. Since tensile stress is equal to tensileload divided by area, the tensile stress in the .44 spl will actually be slightly lower for equal pressure charges.
Obviously this is all based on the assumption that Ruger is using the same grade and specification of steel in both guns. I personally have no reason to doubt this.
This leads me to three conclusions:
If a given pressure level is considered safe in a full size NMB in .45 Colt, I would consider the same pressure level to be safe in the .44 spl flattop.
The .41 flattop and Redhawk have a lot more wall thickness and therefore pressure capacity than the .44 spl or .45 Colt.
S&W is obviously comfortable with lower factors of safety than Ruger, as reflected in the dimensions shown above.
Comments?
To start, I measured various revolvers from my collection using a digital caliper:
-------------------------------------NM Flattop-----NMB Bisley-----NM Flattop-----Redhawk-----S&W 629-6
-------------------------------------.44 Spl---------.45 Colt--------.41 Mag--------.44 Mag-------.44 Mag
Cylinder Diameter:----------------1.675”-----------1.731”---------1.731”---------1.782”---------1.709”
Cylinder Length:-------------------1.610”-----------1.705”---------1.672”---------1.750”---------1.702”
Cyl Outer Wall Thickness:---------0.085”-----------0.076”---------0.099”---------0.117”---------0.078
Cyl Web (between
charge holes) Thickness:----------0.064”-----------0.064”---------0.111”---------0.085”---------0.085”
Forcing Cone OD:------------------0.622”-----------0.622”---------0.622”---------0.685”---------0.625”
Top Strap Thickness:--------------0.248”-----------0.250”---------0.247”---------0.270”---------0.217”
Top Strap Width:------------------0.703”-----------0.720”----------0.713”---------0.705”---------0.665”
Charge Hole Diameter:------------0.460”-----------0.482”---------0.440”----------0.460”--------0.460”
The pressure generated inside the cylinder when a round is fired produces tri-axial stress in the wall of the cylinder. While a gun designer would be concerned with all three stress components, hoop stress is the only relevant consideration for the purposes of comparing the two revolvers. Hoop stress is given by the equation S = (P)(D) / (2t), where S is the hoop stress, P is the internal pressure, D is the mean diameter, and t is the wall thickness.
Since the wall thickness varies, use the minimum value. For the .44 spl, t is 0.065” at the web and D is 0.460+0.065 = 0.525”. For the .45 Colt, t is 0.065” at the web and D is 0.482+0.065 = 0.547”. If we use the same pressure load in both revolvers, the hoop stress is:
For the .44 spl: S = (P)(0.525) / (0.130) = 4.0385(P)
For the .45 Colt: S = (P)(0.547) / (0.130) = 4.2077(P)
Comparing the two, we see that for equal pressure loads, the hoop stress generated in the 45 is 4.2077/4.0385 = 1.042 times the hoop stress generated in the 44 spl. While the cylinder bolt notches definitely cause a stress concentration, they can be assumed to affect both guns equally for this comparison.
The forcing cone diameters of the two guns are the same. The top strap is slightly smaller on the flattop with a cross sectional area of (0.248)x(0.703) = 0.1743 in2 compared with the full size NMB at (0.250)x(0.720) = 0.1800 in2. The tensile load induced in the top strap will be proportional to the square of the inside diameter of the cases. For an equal pressure charge, the tensile load in the top strap of the .44 spl will be (0.430^2)/(0.452^2) = 0.905 times that of the .45 Colt. Since tensile stress is equal to tensileload divided by area, the tensile stress in the .44 spl will actually be slightly lower for equal pressure charges.
Obviously this is all based on the assumption that Ruger is using the same grade and specification of steel in both guns. I personally have no reason to doubt this.
This leads me to three conclusions:
If a given pressure level is considered safe in a full size NMB in .45 Colt, I would consider the same pressure level to be safe in the .44 spl flattop.
The .41 flattop and Redhawk have a lot more wall thickness and therefore pressure capacity than the .44 spl or .45 Colt.
S&W is obviously comfortable with lower factors of safety than Ruger, as reflected in the dimensions shown above.
Comments?