Hmmm, I wonder if the .002 "jug" is intentional or just something that occurred on that set? Planning a .002 jug in a .050ish choke seems insignificant.

Backboring about .010 may have been the thing that allowed the .010 tighter than normal to work without blowing the pattern??

I'm wondering if the overbore allowed for some additional(has to be some in any gun/shot) gas blow-by that caused the funny sound?