The western slope is better suited for random off the road shooting. The front range is mostly private property and even then if you find a forest service spot there are so many homes, hikers and so on around that it is likely someone will call the police on you. The best bet is to find a secluded spot up at the Pawnee grasslands. Look up the forest service website as there are areas of Pawnee that are closed to shooting permanently as well as the occasional fire danger closures. Get a map as well to be sure that the spot you are on is public land as there is a fair amount of private property even within the grasslands. Be sure you have a good backstop and that there aren't any random cattle over the next Hill. Carry equipment to put out a grass fire if one should start.