Thin walled brass barrel with a sliding compensator thing? Sounds like a PPS barrel with one of their speedball muzzle breaks on it. They were designed to slide back when you needed to be small in a bunker and slide foreward so you can have a longer barrel to push bunkers with.
"There's nary a beast that can outrun a greased-up scotsman!" - Willie
"What makes a VM so charming is that you can chop down small trees with it" - Jaan