A-10 Warthog | Davis-Monthan Air Force Base

Julian Hayashi-Marsano adds, "The gun (missing from this airframe) is a GE GAU-8, and it's 30mm. The 20mm refers to a cannon used (chiefly) for air-to-air engagements, the GE M-61 series.
An excellent tank-killer, at least when operating in permissive air environments. Those Depleted Uranium (DU) rounds cut armor open quickly. The recoil is so intense that a 1/1.5 sec burst can bleed (perhaps I should say 'knock'?) 100 kts of airspeed, so pilots are trained for top-down attack at a shallow dive.
I've always loved the Warthog and its simple-is-best design philosophy. Its tortured in-service history is a testament to the benighted leadership of the Pentagon at the time, especially in the AF.
Unless I'm mistaken, there are no Active squadrons flying Warthogs currently, only Guard units--so this is an aircraft that benefits from being flown and maintained by seasoned pros who love their work!"
Sandy Duffy added that the port engine is missing and the starboard side is visible, and Rexamillion Aquino points out that the NF on the tail is for "Niagara Falls". Finally, Kris Johnson added a link so you can listed to what the guns sound like in action, link here.
Thanks for the info, folks! I know nothing about aircraft so the community contributions are appreciated!
Photograph taken at AMARG, the 309th Aerospace Maintenance and Regeneration Group at Davis-Monthan Air Force Base in Tucson, AZ by Matthew Christopher of Abandoned America.