Just found this thread... It was my Franken-FAL. (Are you sure you didn't see it on FALFiles?)

The unwanted but equally entertaining burst firing was cured by cutting the chamber (& hence rim recess) slightly deeper. The bolt hitting the back of case occasionally set one off! Was eventually diagnosed by looking at the back of the fired cases...

The barrel was an Anshutz that had been re-purposed from it's original home. No idea who built it originally, but I'm about to embark on the same project again... (A friend of mine now owns the original one.)