Conflicting info on the SOCOM seems to be the norm, I still dont know what the inner barrel length...
Are you looking to do something like this?
If so Echo 1 sells a pretty cheap Rail system for the M14 that is sturdy enough for airsofting, but I have to admit the other kits look alot better...
Here are the photos I took of mine when I upgraded the bushings and spring:
Sadly the motor is covered up...
I'm pretty sure it uses the same motor as the standard M14 though.