I had some centerfire .22 handguns and was playing with cast. Namely the old Lyman round nose 48 gr. GC. It was basically a CF version of the nominal .22 RF 40gr. pill. Performance was lackluster, even at 2000 fps. Like shooting an ice pick.
I got a goofy idea to use the LEE .224 sizer die to hold the bullet centered and using a rod with a machined tip to slide down the middle of the die "cold forge" a HP cavity into the homely old bullets. Paco Kelley made that .22 RF tool that gave me the idea.
By golly it worked like a charm. The control was me custom cutting the rod to be flush with the die top (where I could not tap it deeper)at exactly the point I wanted it. It was too simple.
I used my .218 Bee rifle as a control. Running both RN and HP bullets at 2200 fps and shooting soap blocks as a consistency control it proved at least a doubling of energy transfer!
I didn't need a new $150 mould, only a 3" piece of brass 1/4 dowel.