I need some help identifying the barrel in the photos. It's similar to an AR15/M16 barrel, but the gas block/sight is unlike any I've ever seen. It doesn't take a traditional AR15 gas block with roll pin and it appears a piston of some sort attaches to the outside of the output coming rearward. There are gas rings on it. The spring-loaded screw on the right side turns 90 degrees. There's also a hole in the bottom of the extension that appears to be a pressure relief / blow hole. There is no index pin on the top of the barrel extension. The barrel length (bolt face to end of threads) is apx. 19.75". A 5.56 GO gauge goes into the chamber as does a loaded 5.56 round. It appears to be 22 caliber. Gas block journal is apx. .625". The flash hider comes back over the shoulder of the threads.
The number "184" is on the block and the barrel. There's also an "N" on the barrel in front of the gas block. Just in front of the extension, there's a "P" inside a diamond. In the same area there's also what appears to be an "M 4".
pics here: http://www.firearms4less.com/pics/unknownBarrel/index.html
Do any of you have any idea what this is?
Thanks in advance.