The same thing has happened to me. I have a brand new .243 Handi (2004 mfg., purchased in January 2005). It has unlocked several times during a total of 80 rnds fired. It did so once during my last range session on Saturday when I fired ~30 rnds.
A couple of the 80 fired cases have primers with drag marks on them.
And, yes, I have cleaned the heck out of the gun.
That said, the issue seems to be getting better as the gun gets broken in. It may be a "new gun thing." (I notice that even though your Handis are 9 yrs old you say they've been shot very little.)