The site I was thinking of is a side road marked by a fence right before it (traveling southbound on CO-126).

It is a nonresidential camping area in Pike National Forest with several miles of dirt roads. If you keep to the right on the road and drive 1.5-2 miles (turn right at the fork), the road will end at a hill. The spent shell casings mark the spot.

The approximate address off of CO-126 is:
22228 County Road 126, Sedalia, CO, United States

It is in Pike National Forest, but I have never had or heard of any trouble from the U.S. Forest Service for shooting there.