I was talking with a instructor at RBC comparing notes on AR .22 conversion accuracy. The problem is the 223 barrels are to big. Accuracy Speaks will put a .22 match grade barrel on your dedicated AR upper solving this problem.
http://accuracyspeaks.com/public_html/22lrar15.htm
The owner, Derrick Martin is a long time National Guard shooter and a great patriot. He is now building very accurate guns.
The problem may be ammo (brand/bullet velocity).
My CAR15 will shoot dime sized 25 meter groups when I do my part. That is, my CAR 15 with a .22 rimfire conversion kit and Remington golden bullets. The CAR15 has a very short barrel as compared to most AR's.
To each his own, use what works best for you.
Barrel Twist is HUGE for the conversion kits.
Please see topic below.
It's in the section of the trial board in this section if the link doesn't work.
http://appleseedinfo.org/smf/index.php?topic=8898.0
sk
I think a lot more is made of the barrel size than really matters; twist rate (as pointed out in other threads) may have more importance.
I use my conversion with my standard 1:9 .223 upper (20") and get consistent 4 moa groups shooting prone using CCI blazer ($19.99/500).
Finding an ammo that shoots was the key - soft nosed blazer did it for my ceiner - it didn't like the plated stuff (federal etc.).
I can't justify in my mind/wallet a dedicated .22lr upper when I can shoot 4moa.
You can spend more on a dedicated .22lr upper than a decent .22lr that is really accurate.
The issue is rate of twist, not barrel dia.
Even so, I have seen some VERY nice rifleman standard targets that were shot with a .22 conversion.