i like the idea of the 500 jrh...how about a 510 jrh?

thin the 500 brass out a bit and stuff with a 510 bullet.

could you use the same 500 dies if the neck thickness is the same?

The neck thickness is around .013-.015" ish. So to inside neck ream .005" from the wall would leave things too thin(IMHO). The brass would be weak and may crumble seating a bullet or may not hold the bullet well enough when crimped.
500 Linebaugh brass and dies are availible. The dies are more as they are not a stock item.
There is always the 50 Special (.510)

do you have any info on the 50 special?

Check out www.reedercustomguns.com He has a 510 GNR that sounds like what you are looking for.

o.k so the 500 jrh and the 510 gnr are the same length?
if so that is cool.
i as thinking about trying to use the more avaliable 500 s+w brass but this does seem to make the most sense...

i was thinking that the brass was thicker at the 1.2" part of the s&w brass enough to thin it down to accomodate the 510 but if i am reinventing the wheel...

Boom Stick, The Trim To length of the 510GNR brass is 1.250". I don't know anything about the 500JRH. Brass for the 510GNR is available.

I do believe the 500 JRH is 1.4" like the Linebaugh 475 & 500. As much as I liked the 500 Linebaugh, there are a lot more bullet options in .500" by most of the big bullet makers, where the .510" is pretty limited. Now if you cast, it should really matter. You can easily trim 500 S&W for the 500 JRH, but when I use to make 500 Linebaugh brass from hard to get 348 brass, I had a lot of blisters from especially inside neck reaming the thick brass. Lot of 500 Linebaugh brass now days though.