Find the bullet with the highest ballistic coefficient and launched at the highest velocity.

There are a couple of cartridges out that use a 20mm cannon case shortened and necked down to accept 50 caliber bullets. If I remember correctly they will launch a 750 grain Hornady A-max bullet somewhere around 3500 - 3600 fps.

Two that I know of are the 50 McMillan Fat Mac and the 50-20 Pac-Nor improved.