Gotta disagree on that , quick google search from multiple manufacturers say 304 , here is a clip from spikes themselves.
Spike's Tactical has produced what we believe is the best gas tube out on the market for the AR-15 rifle. Our gas tubes are made out of Mil-Spec 304 stainless steel and then are Melonite coated for greater heat resistance, corrosion resistance, and lower reflection. These gas tubes are not only great for .223/5.56 uppers, but are perfect for the 5.45 uppers when you shoot cheap, corrosive, surplus ammunition. These are the same gas tubes that we install on our ST-15 rifles and all of Spike's complete upper receivers.