Green Mountain only uses 1137 in their black powder octagon blanks such as barrels for Hawkens. All the others are 4140 except AR barrels are 4150. All current Knights are either 4140 or 416 and ive never seen them listed any other way. The Knight barrels ive bought from GM that had the receiver cut off are 416 and 4140 for sure. Calling 1137 rifle grade steel would be somewhat of a stretch. Nobody ive seen uses it for centerfires. That is how the Revolution is listed in the old catalog and manuals.
http://www.krwarranty.com/wp-content/uploads/2014/07/inline_6_29_11.pdf
All the barrels in the old manual are listed the same way.
Barrel:
27", Green Mountain Rifle Barrel, Rifle Grade Blue or Stainless Steel, 1:28 Twist, .50 caliber
27”, Green Mountain Rifle Barrel, Rifle Grade Blue or Stainless Steel, 1:26 Twist, .52 caliber
As you can see, 4140 is far stronger than 1137
12L14 Tensile strength 78,300 PSI, Yield strength 60,200 PSI
1137 Tensile strength 108,000 PSI, Yield strength 76,100 PSI
1144 Tensile strength 108,000 PSI, Yield strength 89,900 PSI
4140 Tensile strength 148,000 PSI, Yield strength 95,000 PSI
Green Mountain offers more than one kind of barrel rifling. They offer cut rifling that in many cases is better than button rifled. All Mcgowens are button rifled as far as ive seen. Like i said, i would call Cecil about the barrels he is selling. Those might be cut rifled because its a non standard GM twist for 50cal. Its simple to change the twist rate for cut barrels but for button it requires buying the button.
My personal opinion is that GM barrels are at least as good as McGowen and possibly better if its cut rifled. A heckuva alot of ML matches have been won using GMBs. One of the USA made Sharps companies is even using them due to lack of available Badger barrels. They could have easily gone to McGowen if they thought they were a better barrel.