Interesting... Good Question.
It says Chrome Moly in the upper description, but the lower detailed description says, "Barrel: 16" chrome moly vanadium steel chambered in 5.56 NATO, with a 1:7 twist, M4 barrel extension, mid-length gas system and a light weight pencil profile. The barrel is nitride coated and finished off with an 0.625" diameter F-marked front sight base with sling swivel, classic hand guards and an A2 flash hider."
I just purchased the same upper last month and is a first Freedom line upper for me. Bought it for a build for my brother. Nice upper but I thought it was 4150.
Doesn't really matter anyway, it will still shoot fine. Have a S&W M&P15-OR with a 4140 barrel and it's a nice shooter. Probably won't ever put enough rounds through it to wear it out.
As long it's a straight shooter, I'll be happy.
Looking at all the different nitrided Freedom uppers, the descriptions all vary in the descriptions between Chrome Moly and Chrome Moly Vanadium.
But still would like to know also.