If greater than .50, bulles must weigh a min. of 210 grains.
From .40caliber to 50 caliber, bullets must weigh min. 170 grains.
There has never been any rules about it being lead bullet only. Where are you getting your info? Im reading the manual right now and find nothing about that.
shotshells, primers and flintlock are legal.
Also have found no barrel length limit.
Just looked on the colorado DOW website and there are no rules about it having to be an all lead projectile. Scroll down to page 3 and the new rules are in there.
http://wildlife.state.co.us/NR/rdonlyres/6E977561-C613-466D-BCFC-2CA9C9C91CD9/0/Ch02.pdf