Oops. The WE model I linked earlier is a gearbox model, not a gas blowback model.
It's the gas blowback models that are convertible, supposedly.
http://www.airsoftgi.com/product_info.php?cPath=139_482&products_id=9461I don't see any more WE branded ones that are metallic receivers with gas blowback action. Not sure on the other brands. But the metal receiver ones are quite a bit more expensive than the plastic receiver ones.
ETA: The Inokatsu M4 GBB rifle looks amazing. Literally, all I think it needs to convert to a true rifle is a trigger kit, proper buffer spring of correct strength, a correct bolt to replace the aluminum one in the BCG along with firing pin (might need a full BCG if there are other differences), a 5.56 barrel, gas block, and gas tube. But, it's $900+ everywhere I look.