Chris,
Today, many folks don't appreciate the intelligent use of available materials made by designers and manufacturers during the nineteenth century. The progress in understanding steel chemistry and in all phases of steel manufacture and use during this time was remarkable, moving in the arms industry from (in all but best guns) from rather rough and ready materials and methods to the readily recognizable.
I have heard many remark on the lack of control of impurities in steel during this time. To lay this notion to rest obtain a period razor, etch it, then examine the structure of the steel under a microscope; you'll be amazed at how clean a steel was produced when needed.
For production guns like the Remington New Army and the Colt M 1851 (Navy), the steel was much like the steels in use in all U.S. made handguns until just after WWI, a low to medium carbon variety with minimal alloy content. Most handguns of this period had few heat treated parts (excluding the obvious, like springs).
Today, most blued handguns, including the reproductions are made with medium carbon steels. Alloy content and heat treat vary considerably from manufacturer: nowhere is this variety more apparent then in the reproduction arms business. Generally steels like 1030, 1045, 4130, 4140, & 4350 are used for the frame and barrel.
Better quality stainless guns are made largely of the 400 series. 416R (and variants) are typically used for barrels, softer parts sometimes use the 17-4's or 18-8's.
On the other hand, forging has just about disappeared as a manufacturing process in the arms industry. In some applications, this allowed lighter weight parts to be used than do some of the more current methods like investment casting, MIM, and the powder metal family.
Yes, metallurgy has made great strides since 1855, but as a mechanical engineer you'll not find me stating that the Colt or Remington product of the mid nineteenth century is either better or worse than the typical reproduction of today. Exception (in part): the Ruger Old Army model is stronger than any of the old handguns, including the reproductions. But the Colt and Remington originals had hand fitting absolutely lavished on them, even during wartime production. With a background in machine work you'll readily appreciate that proper fit up can overcome a multitude of minor "sins".
Had the guy at the show been discussing "high end" handguns made in the U.S. from 1935-1940 he might have had a valid point. The manufacturing controls insofar as materials and process resulted in a product that approximated the strength of todays production handguns (not, for instance, a Freedom Arms M83- this is a semi-custom piece), and the hand fitting had not disappeared. If you have the opportunity, shoot and examine a Colt Shooting Master or S&W (registered) Magnum. Even H&R made a remarkably fine shooting pistol (the U.S.R.A. model) at this time.
One last note. Above I referred to the Remington New Army. You referred to this as a M 1858, I think. The M 1858 properly is the Beals revolver, an arm which has never, to the best of my knowledge, been reproduced (no matter what Lyman and others have to say). The Beals model has no notches between the nipples, it has a German silver cone front sight, the web below the barrel through which the rammer passes is much smaller than in later models, and the patent marking is different than on the New Army. The marketing folks didn't do their research.
regards,
Bob