FYI Transporting an AR15 through Denver CO...

[Dan W]

...Is legal as long as you don't possess any magazines with a capacity of more than 21 rounds. 

A 30 round magazine magically converts your semi auto center fire rifle into an "ASSAULT WEAPON"   

http://www.rmgo.org/gun-law-faqs/assault-weapons

Can I legally possess a semi-automatic,  so-called "Assault Weapon" in Colorado?    CONDITIONAL YES. Some municipalities ban them or require their registration.  Though SB03-25 was  designed to remove these municipal restrictions, a November 2004 ruling by Judge Meyers  kept the Denver ban in place (though modified).
  Note: In January 2007, the Denver City Council passed  38-07, which modifies Denver's assault weapons ban. 

What is an "Assault Weapon"? Colorado law does not define an "assault rifle" or  "assault weapon". for a short video that may help you understand the difference between a full-auto rifle and semi-auto rifle (Note: the narrator of the video misstates  that fully automatic firearms are illegal -- please see our NFA  weapons FAQ section.
 

Does the City of Denver  have an Assault Weapons Ban (AWB)? YES.  Section 38-130  of Denver’s Revised Municipal Code was amended in  2007 and is titled “Assault Weapons”  Denver's Assault Weapon ordinance (as well  as other firearms ordinances) can be  viewed here.  That ban, however, only applies to firearms with magazines  that hold more than 20 rounds.

NOTE: This means it  is legal to own an AR-15, FNFAL, HK91, MAK90, etc, as long as you do not insert  a 21+ round magazine.

Does Denver’s AW ban  specific firearms?  NO. Denver had a list of specific  weapons that were banned but the 2007 amendments eliminated that list.  For  example, Denver’s old law banned the Colt AR15 by name.  That weapon and all  others on the previous list are now allowed in Denver with certain exceptions –  namely those weapons with a high capacity magazine of 21 or more rounds.  So, an AR15 with a 20 round magazine is not considered an "Assault Weapon" by Denver's  ordinance.  That same rifle with a magazine with a capacity of 21 or more rounds  is considered an "Assault Weapon.".

Does that mean that I can  own a so called Assault Weapon in Denver? CONDITIONAL YES. There are limitations on what you can own.  Generally, Denver’s new law bans the possession of both high capacity magazines of 21 rounds or more and all  rifles with a magazine of 21 or more rounds, which makes it  an "Assault Weapon" in Denver's eyes.

 Does Denver ban shotguns as  well?  YES. Any semi-automatic shotgun with a magazine  capacity of 6 or more rounds or a folding stock or both is also considered an  AW.