Myke, there's tons of rattlers from Pueblo on down south to New Mexico. Any of the SWA's along the front range should be good if their rocky and brushy. I would advise Lake Pueblo SWA in the back of the reservoir, Lathrop State Park down by Walsenburg, or drive Burnt Mill Rd out of the south side of Pueblo which runs to Colorado City. It's a dirt road which I always see either live rattlers or dead ones people have run over on. Both of the lakes I mentioned are surrounded by rocky, cactus and brush filled areas which hold a lot of snakes period both rattlers and other species.
ecu, you can find em up to I believe 9,000 ft although, I haven't seen any that high. The most I've seen have been where the flat eastern plains meet the foothill rolling hills but up to the base of the mountains. Rocky cactus filled country which is hotter then hades in the summer.
What does "gauge" mean anyway? As used here gauge means the number of round lead balls the same diameter as the inside of the barrel (aka the bore) that it would take to weigh one pound. It takes 12 lead balls the same diameter as a 12 gauge barrel to weigh a pound. The smaller 20 gauge would require 20 balls of that barrel diameter. The larger bore 12 gauge would require 12 lead balls of the bore diameter to weigh one pound.
In general the number of pellets in a shotgun shell...