Howdy Again
The pressure vessel in any revolver is the cylinder. It is the cylinder that contains the pressure when the cartridge fires. The frame merely supports the cylinder. The frame does have to put up with the pounding generated by recoil as the cartridge is shoved back against the frame, and with a heavy recoiling revolver the frame has to put up with the vibration caused by heavy recoil. You often read about the early S&W 44 Magnums that had problems with the frame because of the pounding of the heavy recoil. You might even read about frames stretching under heavy recoil. All of this is true, but it is the chambers of the cylinder that have to contain the actual pressure generated when a cartridge fires.
Here is a photo of an antique Merwin Hulbert revolver that blew up when it fired a cartridge too powerful for the old steel of the cylinder to contain. You can see the top strap is missing. When the cylinder ruptured, the gasses and fragments of steel hit the top strap and broke it off the frame.
Here is the cylinder. You can see what actually happened. The thinnest cross section of the cylinder is the section of steel under the cylinder locking slot. That is the point where the rupture started. Many shooters believe cylinders rupture first at the thinnest section of the walls between adjacent chambers. But what usually happens is the steel lets go first right at the locking notch. Then the rupture propagates for the length of the cylinder. In this case, the entire top half of the chamber lifted off, and as it flew upwards it hit the top strap and tore it off the frame.
You can also see the two adjacent chambers were pinched right at their locking notches, deforming them too.
In more severe situations the walls separating the adjacent chambers will also rupture and half of the cylinder will lift off, leaving three chambers ruptured.
In your case, as long as the guy boring the chambers out for the new cartridge does it correctly, and keeps everything lined up perfectly, opening up the chambers to 44 Special should be no problem. If he screws up and does not do it properly, all bets are off.
Here are two more photos that should help explain things. The cylinder on the left is from a Colt SAA chambered for 45 Colt. The one on the right is chambered for 44 Special. Both cylinders have the same outside diameter. But notice how much more steel surrounds the chambers in the 44 than the 45. I suspect that is why the SAAMI standard for Max pressure with the 44 Special is a little bit higher than the Max pressure for 45 Colt. Bore bigger holes and you leave less 'meat' surrounding the chamber.
There are other things involved in the strength of a cylinder than just the amount of metal. Most modern cylinders are heat treated for strength, and have been for many years. The cylinders of old revolvers were not heat treated, so that enters into the equation too.
SAAMI sets the maximum safe pressure for 44 Special at 15,500 psi.
I recall seeing somewhere the .357 was SAAMI rated around 35,000(??)
BSA1 above mentioned that I might consider having the .357 cylinder bored out to the .44 size, if I have the 357 cylinder bored out to the .44 size have I gained anything in the way of added strength?
Yes, the operating pressure of 357 Mag is much higher than 44 Special or 45 Colt. Look at it this way. All cylinders made of modern steels start out with pretty much the same inherent strength. It has to do with the tensile strength of the steel itself. By keeping the holes smaller, you remove less ability for the cylinder to withstand high pressure. The other thing is, SAMMI Max pressure specs for cartridges like the 44 Special and 45 Colt are purposely kept on the low side because there are still a lot of old guns out there that are not made of modern steel, and might not be able to stand up to higher pressures.
This photo may help further explain things. Left to right the cylinders are an Uberti Cattleman, ruger 'old model' Vaquero, and 2nd Generation Colt. All are chambered for 45 Colt. Notice how much more metal there is surrounding the chambers in Ruger cylinder than the other two. The Ruger cylinder is slightly larger in diameter, and the chambers are spaced slightly farther apart. That is the reason for the so called "Ruger Only" loads you will find in some reloading manuals, and it is the reason why you don't try them in a Colt or Colt Clone.
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Regarding your frame: did you take any measurements before and after removing metal? Do you know exactly how much metal you removed? The reason I ask is Colt SAA frames are Case Hardened. They are not heat treated all the way through like for instance a Ruger frame is. That is the reason for the beautiful colors on a SAA frame. But Case Hardening is a surface treatment, it infuses extra carbon into the outer surface (case) of the frame, creating a hardened layer on the outside. This hardened layer only extends a few thousandths into the steel, underneath the steel is more ductile. No, it should not affect the overall strength of the frame, remember the frame only supports the cylinder. But you will probably notice if you go to engrave the frame that the surface will be softer on the side where you removed metal than it will be on the other side.
Regarding Max loads: I am not suggesting you approach Max loads. I NEVER approach Max loads when I reload cartridges, I always stay comfortably in the middle ground between min and Max loads. Too much chance that a little bit of inaccuracy with the powder measure might kick the load over beyond the Max load, and besides, with Max loads you get Max recoil and I don't like that. I only mentioned it because you asked about 1000 and 1200 fps loads. Those are going to be Max loads. With some cartridges and powders, best accuracy is obtained up at the Max load level. I strongly suggest you stay down at the starting level and middle when you start reloading. Very few reloaders have pressure taking equipment available to them, you have to learn to look for the signs of overpressure on the cartridges. That's why you are going to buy a really good manual that talks about that kind of thing.
Here is what a good, experienced reloader will do when he starts to reload for a new cartridge/gun. First, reading the available data he will make some educated guesses about what type and weight of bullet he wants to load and what powder to use. Then he will load up some cartridges starting at the lowest level in a few increments of maybe .5 grains. Maybe ten loads at the minimum level, then another ten at min + .5 grains, and so on. Then he goes to the range and sees what load performs the best, either in accuracy or penetration or what ever he is looking for. Then he will go back to the bench and further refine his loads, until he arrives at the load he is most satisfied with. A poor reloader will look up the data, load up a box of Max loads, and take them to the range and blast away with them. Reloading is a whole hobby in itself. It is done for a variety of reasons.
One more thing about reloading. You don't save any money reloading. You just shoot a whole lot more for the same amount of money over buying factory cartridges.
