Places to shoot at least 500yds?  
Google found Adair range - any other options and is it open?  
What about federal land shooting?  
Interweb says no pow range on post?

I took my 50 to the un-ironically named firing range off of firing range road.  The grid for the turn off of Route 95 is 32.712020231998224, -114.42833118103292, and the approximate area is 32.698913262980376, -114.4042803924836.  I'm pretty sure it was a WWII range, but now it's nothing.

There is nothing there but rocks and desert and a mountain to shoot into, but it's close to town and easy to get to, even in a car.

about 20 years ago a group of us went to shoot an NRA-style high power rifle match at Adair range. It was a 1000yd range as I recall, we shot 200-300-600yds.

I have no idea if they still run matches there, a google search shows the range is still there 4760 S Us Highway 95, Yuma, AZ 85365 and the rifle range road one is called Blaisdell range.

check them out and report back to us.

Poole

Man, I would not want to be shooting down there. Too hot even for me