I was reading a massive screed about barrel steels for the AR today. Colt has finally submitted its Chrome Vanadium Molybdenum steel formula to ANSI for standardization. It's going to be called 4150CVM. It can be made a bit harder than 4150 which can be made a bit harder than 4140.
EDIT: 4140 has less tensile strength (655 v 729.5 Mpa) but a higher yeild strength (417.1 v 379.2 Mpa) than 4150. Yield strength is: Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible. Tensile strength is: the stress at which a material breaks or permanently deforms.
My M16A2 clone uses and the XM177E2 will use 4150CVM because that's what Bushmaster ships nowadays. My R604 clone uses 4150, because that's what Colt used in 1969. All of my other guns use 4140 because that's what E.R. Shaw and Wilson Combat (Cardinal and Del-Ton respectively) use because it's much easier to machine correctly and for non-automatic, non-arctic use it's perfectly capable of doing anything the other two steels can do. 4150 is more resistant to heat changing its composition than 4140. 4150CVM has a finer grain structure than 4150 that allows it to better resist cracking and splitting when hot and exposed to extreme cold.
The heat we are talking about is along the lines of firing several hundred rounds without letting things cool off. Extreme cold is North Atlantic salt water in December on a barrel that has fired 500 rounds without stop.
Conditions that the average user will never encounter. 4140 is very similar to the steels that were used with great success in WW1 and WW2, so it's not completely useless for extreme use, just not as good as 4150 or 4150CVM.
If I was equipping the troops, I would spec the CVM steel. Our men are worth the extra money. For myself, I think I will get cheap and not overheat my barrel when it's inhumanly cold out.
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