A 505 Gibbs available om Gunbroker.com-

http://www.gunbroker.com/Auction/ViewItem.asp?Item=71214087

Don't know about their OOB reliabity but I am sure with everything sorted out it would be a superb DG bolt.

Another option previously discussed in other threads would be to get a CZ 416 Rigby and rebarrel to 500 A Square/500 Wells(460 Wby case with .510 bullets). Brass availability wouldn't be a problem, the cartridge would be somewhat smaller and could be downloaded to 600gr @ 2250 from 2400fps and still be a great DG bolt.

Either gun would be portable at around 10.5 pounds and a great DG bolt.