CW-
We had one of these when I was a kid. Called the High Velo-Pen by the designer, it was intended to pierce metal. According to the designer's writeup, it would go through 3/16' hot-rolled steel at 25 feet using (ADVISORY!! EXCESSIVE LOAD NOT RECOMMENDED ANYWHERE ELSE, EVEN IN THE LYMAN MANUAL IN WHICH THE WRITEUP APPEARED.) 26.5 grains 2400 in a .44 magnum case. He also compared it against 240-grain factory loads (swaged lead SWC) and found it would punch through over 4" of green wood "while the factory load stayed on the inside." IIRC, the Lymman manual I read came out in the late 1950s. Those publications were a bit on the wild side.
Dad carried them, over 24.5 grains 2400, in a Model 29, for protection in the Alaskan wilderness. I fired a couple, and stopped. They were cool bullets to cast, however.
The 429303 saw a brief resurgence during the Clinton Gun Control Panic of 1993-95. I recall molds, primers, powder, etc becoming scarce for several months (Primers went for 50.00 a carton in some markets). After that, Lyman stopped making them and I have not seen them. With the current corps of custom manufacturers, the 429303 is unique, but the capabilities are not as scarce as way-back-when.
Hope this helps. It's been years since I read the article. Maybe Google "44 Magnum High-Velo-Pen."
Bisley