Now i know this has nothing to do with the ATM Hardballers but i know at one point Marushin made a Bren-Ten (out of production now as far as i know) from what i've read they were a pretty decent GNBB and looked exactly like the real thing ( in fact one persons review i read said he put a real Bren Ten slide on it for display :P)
http://www.bren-ten.com/ <---- Bren-Ten dedicated site
http://www.bren-ten.com/id24.html <------ Section on the Airsoft Bren-Ten