talk to QDRenegade at Arnies,
He's put a P12X onto a GR16.
read his posts in this thread (might help a little bit)
http://www.arniesairsoft.co.uk/forum...&#entry1960344
From what he told me when i asked about it, it fully extends over the receiver, and mounts to the top rail. So I'm not exactly sure if you would have to rethread the receiver or not. It doesn't sound like it, but I could be wrong.
"Yep, the Predator rails entend over the receivers rail. In the case of the P##X (P4X, P9X, P12X), the rail goes to the end of the receiver. If there is no X (P4, P9, P12) the rail goes to the second to last notch in the rail. That way you can use a rail mounted iron sight at the correct height for a gas block sight."