Though it is certainly possible that M21 tubes were "heat treated" to a strength higher than "normalized" (which is commonly 'as produced' from the supplier, and is of slightly higher strength than full annealed), I have doubts. If they were "heat treated" for strength, as most of you know, steel strength and hardness go hand in hand.
Nevertheless, even if they were trying to preserve hardness/strength of "normalized" steel, it would be a significant strength advantage. Depending on alloy and foundry processing, "normalized steels in the "chromoly" family can have an increase in yield strength (the point of permanent deformation, not rupture) from about 20% to 100% above full anneal yield strength. However, hardness tracks with this increase in strength. Alloys having "normalized" yield strengths near 100% higher than annealed condition also are fairly hard, up around the low 30's Rockwell C. That's about as hard as a modern rifle action, modern 1911 style frame/slide. On the other hand, some "normalized" steels are pretty soft, around 12-13 on the Rockwell C scale. But these are the steels that have only a slight increase in yield strength over their annealed condition.
