People make that assumption but it's just not true. Even standard factory loads are not full pressure. Cowboy loads are even less, probably more like 9000-10,000psi. Which is equivalent to blackpowder pressures.
CraigC
I would like to know where you got your information about the pressures of Cowboy loads. As I have stated, there are no official SAAMI guides for 'Cowboy Ammo'. No information regarding either bullet weight, velocity, or pressure. Pressure of course being the most important.
The official SASS rules for ammo that is acceptable for cowboy action shooting is that velocity shall be no more than 1000 fps out of a revolver and no more than 1400 fps out of a rifle. That's it, no statements about bullet weights, and more importantly, no information about pressure.
I don't buy cowboy ammo anymore, I always load my own. But I had a couple of boxes laying around (door prizes from big matches). Ten-X 38 S&W, Black Hills 44 Russian and Black Hills 44-40. Nothing printed on the boxes regarding velocity or pressure. Just the warnings that the ammo is intended only for 'Modern Firearms that are designed and intended for firing Smokeless Powder ammunition'.
So I went to the Black Hills web sight and looked up their ballistics for their Cowboy ammo. Bullet weight and velocity were listed, no pressure data. Same thing at Ten-X. Bullet weight, OAL, Velocity out of a pistol, Power Factor out of a pistol, velocity out of a long gun, muzzle energy, Energy at 100 yards, and bullet drop at 100 yards were listed. No mention of pressure at all. Next I went to Winchester. Only bullet weight was listed, nothing about velocity or pressure. Next to MagTech, a well known maker of cowboy ammo. The only data listed was bullet weight, nothing about velocity or pressure. Then I went to Ultramax, another manufacturer of cowboy ammo. Bullet weight and velocity listed, no pressure data.
Then I went on line to the powder manufacturers. Alliant has a section about cowboy loads. Bullet weight and velocity only, nothing about pressure. Then to Hodgdon. Finally some pressure data. But no specific 'Cowboy Loads' section. By interpolation of bullet velocities that would be acceptable for CAS, and only using the starting data, I could come up with some pressure data, but it is in CUP, not PSI, and I don't have a good calculator to convert between the two.
Then I went to my reloading manuals. The only one that specifically calls out 'Cowboy Loads' is my Speer, 13th edition. Again, only bullet weight, minimum charge and maximum charge, and velocity. No information about pressure. Lyman Pistol and Revolver Handbook, 2nd Edition (the one I cut my reloading teeth on) no specific CAS recipes. As far as coming up with CAS loads from the standard loadings, again, no pressure data. Richard Lee: he only lists pressure for maximum loads, nothing for beginning loads which is where the CAS ammo should be. Lyman Cast Bullet Handbook - 38-40 no pressure data, 44 Special, yes, there is pressure data, 44-40 no pressure data, 45 Colt no pressure data. Lyman 49th Edition, yes there is pressure data that can be coupled with ammo that meets CAS requirements, but no official Cowboy Ammo loads. And both of these Lyman manuals only list pressure in CUP, not PSI.
So, not to dump on you, because I know you are a knowledgeable shooter, but where did you come up with the 9000-10000 psi figure?
Regarding pressure printed on boxes of manufactured ammo, I was just trolling with that. No manufacturer is going to print the pressure data on their boxes. That is private information that is the intellectual property of the company. They are not going to print it for liability reasons.
Bottom line, there is very little actual pressure data available for 'Cowboy Ammo, either manufactured or hand loaded. Without having a clue about pressure, I do not recommend shooting such ammo in antique revolvers.