The original factory .38 S&W BP loads with a 146 grain lead bullet and 14.1 grains of BP had a 25' velocity of 635 ft/sec from a 5" barrel. Current smokeless .38 S&W factory loads (except those from Buffalo Bore) generally have MVs below 600 ft/sec, and are safe to use in the older black powder guns. .38 S&W ammunition has been factory loaded with smokeless powder since the very early 1900s, and most, if not all, of the old revolvers have used nothing but smokeless powder ammunition for the last 100+ years with no damage. I wouldn't trust any of the old ones with the Buffalo Bore ammunition, but it should be OK for later side swing solid frame revolvers. It's no trick to handload .38 S&W ammunition to produce .38 Special ballistics in newer revolvers, such as the British .38/200 Victory models.
The main problem with modern factory .38 S&W ammunition is finding it.