It's a blued, fluted, 1:9" barrel for shooting sub-sonics from Green Mountain Rifle Barrel Co. I had one once for shooting Aguila 60 grain Sub Sonic Sniper ammunition but it slipped away. This will probably go on my 96/22 lever action which takes the same barrels as the 10/22. While poking around on the site I found that Green Mountain are again carrying the "Stinger Chamber" 10/22 barrels with a 1-18" twist. I have the prototype barrel and have it on my 10/22. It is the most accurate 10/22 barrel I have ever fired. (except with the 60 grain SSS!)
My 1:9" should be here next week.
Normally I wouldn't bother posting this in a UK site but I know that Green Mountain ships to the UK. You can't buy better for the money.~Muir