Originally Posted by
C1PNR
I don't have a Marlin microgroove rifle, so my comments are a result of reading a LOT of earlier posts on this topic.
Brad has it right.
A really good search of this site (yeah, I have trouble making it work for me, too) will bring up many conversations on this very topic. The consensus, IIRC, is that a fatter boolit is the answer.
As you know, every barrel is a law unto itself so what works for someone else may not work for you, but try the fatter diameter before contacting a barrel maker for a replacement. Maybe borrow a mould or ask someone to provide you with some samples.