There's a 600yd in New Holland Pa and another in Bridgeville DE. These are both private ranges. I'm not sure of what their policies are regarding non-members and 50 caliber. It's my understanding that it's very hard to find ranges that will allow 50's.
I doubt that Central Jersey is going to allow you to use their ranges. They run a very tight ship.
Your best bet may be to find a 50 BMG forum and ask there.