In addition to the excellent information provided by Mr. Horton one must also look at the ordinance steel used by the various countries and classes of arms. 4140, 4145, and 4150 ordinance steels are the three most common. 4140 is the cheapest, easiest to machine but has the least resistance to flame cutting and erosion. 4150 is hard to machine but has a very high resistance to wear, heat, and erosion. 4145 is a compromise. 4150 is spec in weapons with a high cyclic rate of fire. A clear example of the difference of these steels can be seen and felt by trying to deflect the barrel of a unstocked Garandicon and an unstocked Enfield. The Enfield barrel will deflect much more than the Garand. I do not have the spec info for the Enfield but perhaps the good Captain will weigh in with the number. I am thinking it maybe around 4140 or a bit less, 4135. 4150 is by design much more immune to the effect of gas leakage around boat tail bullets. Another issue that comes up with 2 land 2 groove barrels and BT bullets I think has more to do with dimensions, bullet upset, and bearing surface. Of the limited number of these barrels I have examined or been given the info on they fall into the dimensions of .3055 to .307 bore and 316 to 318 groove. Even for a bullet as large a .312 the full bearing surface is needed for there to be enough material to be upset to fill the .316 to .318 groove. If a BT is used the amount of available material to be swedged to fill the groove is reduced and blow by happens. With the colder burning powders now used this may not cause erosion like that of the past but the blow by cuts the bullet jacket and more importantly vents the muzzle beside the bullet causing pressure zones that tip the bullet off axis and cause erratic bullet flight.