I have shot the HK MP5 looking one and it was ok. I think the company that makes the AK and MP5 is also making the AR-15 M4 for Colt. If it were me I would stay with a Ruger because they is a lot of support for them. The company that makes the .22 lr knock offs mentioned above is actually an airsoft manufacturer. You might look into the below threads too.
http://www.ar15.com/forums/forum.html?b=6&f=41

http://www.ar15.com/forums/forum.html?b=6&f=11


Men you can't trust, women you can't trust, beasts you can't trust, but Bussekin steel you can trust