Does the letter "s" stamped on the barrel mean stainless? This marking can be found under barrel nut. The barrel is affected by magnet.

[url]http://www.olyarms.com/bblinfo.html[/url]

[B]STAINLESS STEEL BARRELS[/B]

If after the "5.56" markings, if you also see "SS", then your barrel is manufactured from 416 stainless steel. This material is very resistant to corrosion, and is universally accepted as one of the best barrel materials available. Even though this barrel is stainless steel, because of it's carbon content it is slightly magnetic. So if you have just tested your barrel and found that a magnet sticks, don't panic, this is normal.

www.olyarms.com -> TECH INFO -> About Our Barrels :
"STAINLESS STEEL BARRELS
If after the "5.56" markings, if you also see "SS", then your barrel is manufactured from 416 stainless steel. This material is very resistant to corrosion, and is universally accepted as one of the best barrel materials available. Even though this barrel is stainless steel, because of it's carbon content it is slightly magnetic. So if you have just tested your barrel and found that a magnet sticks, don't panic, this is normal."
Walli