The Ruger Old Army can handle smokeless loads...actually I converted a number of uberti 1873 ".44 Blackpowder only) cap & ball revolvers to shoot the .45 Colt and they've had ample amounts of ammo fired from them...including smokeless for the most part.
That could be true, but you don't simply interchange equal amounts of smokeless with black powder. I knew some guys that did that. Started reloading with a 1st Generation .45 Colt. The first round they fired off blew the gun to pieces. Destroyed a very valuable firearm and put a guy in the hospital.
As I understand it, black powder has a pretty linear burn rate. That is, twice as much powder gives twice as much pressure. I'm sure it's not this linear, but it's a LOT closer than smokeless.
I suspect the average shooter has absolutely no idea how the burn rates and pressure curves interrelate with smokeless powder. With smokeless, an increase of 10% or even lots less can increase pressures maybe 100% or more. Using the exact same charge but using powders with different burn rates can have catastrophic results.
On the average black powder muzzle loader (.50 cal), you can vary the charge from 70 to 100 grains as a normal variation. If you vary the charge that much with smokeless, you'll have a bomb.
I don't have a Savage, but as I understand it they give very exact load data, which would make sense.