Thanks a lot, Slim. I think this is going to be more complicated than I had expected.
The ballistic info I find on loaded ammunition always includes bullet weight and velocity at the muzzle; what I cannot find is the pressure, and that seems to be the key to determining if a load is reasonable for the SAA, i.e., ensuring that the pressure is <35,000 psi. So I just now checked a Lee reloading manual, and the table for 357 magnum does show pressure, but not in a way that I can understand. With a 125 gr jacketed bullet, 17.6 gr of Hercules 2400 will produce 1,810 fps and pressure of 31,800 psi; with the identical bullet, 9.6 grains of Unique will produce a lower velocity of 1,585 fps with a higher pressure of 33,800. So, if I understand this correctly (which is unlikely), I cannot determine pressure if the only information I have is bullet weight and muzzle velocity.
So, since pressure is the key to determining the limit of what my Mod.P can handle: how can I determine pressure of a given load?
Can I assume that all available factory loads (from major manufacturers, like Federal, Winchester, Magtech) are within SAAMI pressure limits? If it's more complicated than that, it looks like I'm in over my head.
Apologies for the length and complexity of this reply, and thanks.
Glenn
Howdy, Pard,
Yes, you can assume that factory loads by major manufacturers are loaded within the SAAMI specs for "maximum average pressures". The liability incurred by exceeding those standards would be too great for any manufacturer to bear!
You are correct that you cannot determine pressure based solely on muzzle velocity and bullet weight. The reason is that smokeless powders have different burning rates, and even a single powder will act differently under different conditions. A powder that reaches its peak pressure more quickly than another, may result in a given bullet leaving the muzzle of the barrel at a lower velocity than a slower-burning powder. A slower powder may not reach its maximum pressure before the bullet leaves the barrel.
In point of fact, even the publishers of the loading manuals cannot tell you for certain what a given load will do in any given gun! There are too many variables, for example, the chamber dimensions, revolver throat diameters, brass, bullet seating depth, and a bunch more. What the published data tells you is that that load was tested in a pressure barrel or an actual gun, using the specific components listed, and produced the results shown. Take the exact same ammo, put it in a different gun and the results may change dramatically. What the ammo/powder makers do is to determine a load that, taking into account a margin of safety for typical MODERN-MADE guns, will produce reliable, relatively safe pressures
in most guns. But, change the primer or the brand of brass, and you may have a brand-new ballgame.
You could attempt to measure pressures by purchasing an Oehler M43 Personal Ballistics Laboratory, which uses a strain gage that must be cemented to a chamber of the cylinder. That would show you the pressure-vs-time curve and muzzle velocity. But I can tell you from first-hand experience, there is almost too little clearance between the cylinder and the underside of the topstrap of a Ruger Blackhawk/Vaquero, and I doubt if you could get the gage in the space available in a Colt's. And it would only prove what is happening in that chamber! Plus, you don't know what the particular manufacturer is warranteeing the gun for.
My strong suggestion to you is to read a number of the reloading manuals, including the Lyman 48th Edition, Speer Handbook, Hornady Handbook and the Hodgdon reloading guide, and STAY WITHIN THE RECOMMENDED LIMITS shown in those guides. You WILL find differences in the data from book to book! That is because the test equipment may vary from laboratory to laboratory. When in doubt, take the most conservative approach! You will have...a more harmoneous outcome!
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