[Jastone7373]

Had my HK MR556 apart for cleaning last night and noticed my piston sleeve is kinda funky looking. Couldn't find any other hi-res pics on the net to compare-- does yours look like this?

 

[Crashmc]

Looks like you might need some new rings.

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[Jastone7373]

This was my first range trip. Rings are brand new

 

[Jastone7373]

Plus that's the part of the op rod that doesn't move

 

[mr223a1]

Looks like carbon buildup.