Just buy an AK or a special receiver and barrel for the round. Be sure to have a licensed gunsmith do the upgrade.
Short answer: Not without incurring further expense.
222, 223 and 9mm
If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
Colt has not offered a AR15 in 308
AR15, M16, M4, Ruger Mini 14
If it has a dedicated .223 Remington chamber, you should not run 5.56x45 through it.
$800.00
50-1200 usd
No. Cases are slightly different. You can fire 223 in 5.56, but, you should not fire 5.56 in a .223
Yes. Do not fire 5.56 in a .223
Try looking at this, it may give an idea. http://biggerhammer.net/ar15/ar15serial.html Mike aka nfmMike @ ar15.com forums
The AR15 is a 5.56x45 rifle. Incidentally, it is also capable of chambering and firing the .223 Remington, which is similar in dimension, but the two are not identical. That's the norm. AR15 rifles can be had in a number of calibres nowadays, to include 6.8x43 Remington SPC, 6.5 Grendel, .458 SOCOM, .50 Beowulf, 5.45x39, 7.62x39, .204 Ruger, 9x19 Parabellum, .40 S&W, .45 ACP, 6x45, .450 Bushmaster, and others.
you can fire both 5.56 and .223 ammo in a rifle chambered in 5.56...but not the other way around! In a rifle marked .223, you should fire .223 ammo only