I can change the barrel on my ar15's pretty easy. Ak its pretty hard to do but there isnt much reason to.
With an ar15 you can do it by pushing the two reciever pins out remove the upper receiver from the lower receiver put the new upper receiver on the lower push the two pins in good to go.
If you want to keep the same upper and replace a shot out barrel (10,000-70,000 rounds depending on barrel metal) takes about ten minutes and a barrel wrench.
Unless your running a full auto belt fed there isnt much reason to swap barrels quickly on an individual rifle
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent
Another thing, you can buy two excellent ARs for the cost of the equivocally awesome ACR.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek