You talkin' about Kenny Jarrett's creation or something else?
A Jarrett Catbird is perty straight forward. You just run it into the die until the round will chamber with a tight crush fit and fire away. IMO it's better to use a light bullet and fast powder for fireforming. I start low and work up to a stiff fireform load that shows an ejector mark. This establishes my parameters. Now I back down to about a 90% load and fireform.
hth
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