Try cleaning the throat with Iosso bore cleaner and a stiff Iosso brush.  A lead ring builds up that swages the bullets and ruins accuracy.  JB Bore Shine will also work, but I recommend the IOSSO brush anyway.

Otherwise, bad cleaning practices without a chamber or muzzle guide will wear the barrel out.  If the muzzle becomes slightly cone shaped, accuracy can be destroyed.

The other detail you should check is whether the scope and rings are tight, and if they are, install a known good scope to test the gun.  The barrel probably needs to cleaned in any case.

I just notice you have this in the M-16 forum.  If your upper does not have a rimfire barrel, then you're pretty much stuck with whatever accuracy the barrel produces.  The bore for 5.56X45 and .223 Rem ammunition is too large for .22LR ammunition.

The correct bore is 0.222 inches diameter.  About the only recourse is to find ammunition with bullets that measure 0.224 inches; I wouldn't take on that job, but you might get lucky and find a manufacturer that routinely makes their rimfire bullets on the large side.  I'd start on the cheap end.