Heat the Zamak with a heat gun or oven. It expands at a different rate than the steel barrel. Leave the replacement barrel in the freezer for a couple of days to shrink it. Seems to make things easier.
The ring of fire guns are truly a tale in simplicity. They all follow the basic the same strikefire design with little variations from each company. The only survivors of that era now is high point and phoenix arms. Just because of the race to the bottom in inexpensiveness, there were quality control issues that led to some safety problems that were immediately made into lawsuits. Since the government and anti-gun groups really disliked inexpensive weapons for impoverished people. High point, and Phoenix kept shoving in safety features into their weapons as anti lawsuit protections.
