The N-SSA allows original firearms to be used. I'm not aware on any restrictions in their use - other than the N-SSA does not condone radical modifications of original arms. For example, you can't take an original smoothbore musket and add a rear sight to it if it did not come with one in its original configuration.
Iron in the 1860s was largely made using bloomery steel made in bloomery furnaces. Our regional N-SSA shoot, in fact, is held at Brierfield Ironworks State Park, which is the ruins of a Civil-War era bloomery furnace that provided iron for the Confederacy. Tannehill is nearby and in better condition.
Bloomery iron was produced in this manner for a thousand or more years. Essentially, you make a tall chimney and start a fire in it. Then you dump iron ore in on top. The impurities in the ore (silicates) liquefy, but the iron itself never gets hot enough to melt. If it did/does, the molten iron rapidly uptakes carbon and you end up with cast iron, which is brittle and generally unsuitable for musket barrel use.
https://en.wikipedia.org/wiki/Bloomery
Once the silicates are melted, a cork is pulled out of the bottom of the furnace and the molten silicates and slag runs out of the furnace. It makes a black glass (pieces of it can be found all over the park). Surface tension causes the iron particles to coalesce into a lump inside the furnace - a "bloom". This bloom is removed from the furnace and hammered while still glowing bright hot. The hammering causes any trapped inclusions of slag to be ejected out of the bloom, squirting molten slag everywhere.
The result is iron, with more or less slag inclusions in it depending on how well/long it was "wrought" to to remove and fold/blend the inclusions to make a homogeneous piece of material. In fact the reason why
Japanese swords were folded as many times as they were was to make a more homogeneous piece of metal.
If steel was desired, the way this was achieved was through carburizing. This was known since at least 1200 or so as documented by the monk Theophilus in his work, "On Divers Arts", where he documented the efforts of various tradesmen. One of the processes he documented was the making of files, where he describes packing in carbon-rich material, heating in a forge, quenching, and then tempering. All of the things we know today as carburizing and heat treating.
I believe gun barrels of the era (and earlier) were pretty much just wrought iron. Just post Civil War you started to see gun barrels made of steel and so stamped.
Sorry for the rambling; just a brief history of iron and steel.
Anyway, I have never heard of wrought iron growing brittle from age alone. Metals generally fatigue from stress - repeated flexing. This is why you can take a paper clip and bend it back and forth and eventually it will snap in two.
The reason for antique guns failing in period was likely due to impurities in the iron causing weak spots in them. Or of course, under-engineered parts to begin with. Colt Walker cylinders were notorious for fragmenting under use. It's one of the reasons later designs shortened the cylinder so that less powder could be put in them.