It depends on several things.
Caliber
Brand
Material
If you want a long lasting, decent one. Plan on spending around 1,000$
If you want a cheap, .22 caliber one, Mossberg sells them around 300-400$
$800-$2,000.
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
Colt has not offered a AR15 in 308
$1599
Try AR15.com Try AR15.com
If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
Both are assembly line weapons. Stag in my opinion also has a better reputation. Unless you decide to go tactical, they are all pretty much the same. Bushmaster, dpms, stag ext..
As of today (10/18/08) $900 ~ $1,370 depending on condition.
1959
Yes
By it's serial number. Go to www.proofhouse.com and under 'long rifles' listing look for AR15 Sporter
@ 40 inches.