I think he is talking about a .515 diameter 50-70 bullet. .515s do work in the Taylor, though the Lyman Taylor dies are set for .512s, which is what the Taylor's is designed for. Buffalo Arms has an excellent .512/350 BAC custom mold ($75) that works awesome in the Taylors. I got it instead of the Lymann .515/350, mostly because the Lyman was out of stock. After I got the BAC mold, though, I found that it is the same mold they use to make the .512/350s they sell SPG lubed. This is an awesome bullet, and I had great accuracy with it.
Another advantage to the BAC .512/350 is that it does not seem to seat as deep as the Lymann, allowing for more powder capacity. This is achieved by having a more rounded ogive than the Lymann. With the Laymann bullets, I used a Lee 2.2 CC dipper of Hodgdon's Triple Seven, giving about 37.5 grains (volume) which is equivilent to about 42 of FFG Goex. With the BAC bullet, I can use a Lee 2.5 CC dipper, or about 39 to 39.5 (volume) of tripple Seven. That should equal about 44 grains or so of Goex.
The most Goex I ever got in a 56-50 case was about 39-40 grains of FFG, using a drop tube and heavy compression. The Triple seven gives more bang for the buck.