Howdy
Regarding 41 Long Colt ammunition being currently available:
That depends. When the cartridge was first introduced it used a heeled bullet, the same diameter aw the cartridge case. Just like a modern 22 Rim Fire. Bullet diameter was nominally .408. The chambers of a revolver chambered for the round would have had no 'step' for a chamber throat, they would have been the one diameter all the way through.
In this photo the cartridge in the center is an old 41 Long Colt. You can see how the bullet is the same diameter as the cartridge case. The cartridge on the right is a 44 Colt, which also uses a heeled bullet. The cartridge on the left is a 45 Colt, which always used an inside lubricated bullet that slid inside the case.
You can see the lube grooves on both heeled bullets, you can also see how there is none left.
Heeled bullets were a pain in the butt, they needed to have their bullet lube on the outside of the bullet where it would be in contact with the bore. Because of this, the soft bullet lube tended to accumulate dirt and grit. Ammo makers finally woke up to the fact around 1895 and reduced bullet diameter for 41 Long Colt to about .386 so the bullet could be inserted inside the case, with the lube also inside the case. In order for the small bullet to engage the rifling, the bullet was hollow based so the skirt would expand to engage the rifling.
I am pretty sure any modern 41 Long Colt ammo you buy will have the hollow based .386 bullet. No idea who effectively it engages the rifling.
Regarding shooting an old Colt with Smokeless powder, yes, Colt did not factory warranty the SAA for Smokeless powder until 1900. I never shoot my antique Colts with Smokeless.
Also agree, good luck finding a gunsmith today who is familiar with the old Colts and knows how to work on them.