Pretty tough to find... that's the problem living so close to such a huge city like Denver. I used to go up to the Buffalo Creek burn area (near Deckers) and shoot as far as I wanted, but the forest service shut that whole area down. no more shooting allowed. I have not yet put in the effort to find a new place as I've been too busy the past couple of years starting a family (have a newborn and a 2 year old).
It's been on my mind to find somewhere new to go, and I think somewhere in that general area might still work, but will need to talk to the forest service to make sure where I go is legal. The Colorado trail runs right through there, and at some point I would guess you could into the right locations to legally shoot and have plenty of space to do it.
Feel free to PM me and I'll be happy to share any spots I find, or maybe we can even work together to find something.