Actually the 4 5/8" 44 SBH barrel is a standard offering:
Shown here on Ruger website:
https://www.ruger.com/products/newModelSuperBlackhawkStandard/models.html
Also I have purchased Ruger factory barrels that owners didn't want back from custom pistol smith John Linebaugh:
John Linebaugh - Linebaugh Custom Sixguns
Phone: 307-645-3332
P.O. Box 455
Cody, WY 82414
Delivery (UPS):
177 Louis L'Amour Lane
Powell, WY 82435
http://www.customsixguns.com/index.htm