Well, in terms of AEGS, Tokyo Marui are already doing it with some of the New Generation AEGs with the blowback and recoil already having metal exteriors. But on GBB pistols it's another question though many modern pistols now are actually not made of metal externally. Airsoftology talks about this in this Q&A episode:

"Tippmann Mail Call:

1:20 - What would it be like if Tokyo Marui made metal airsoft pistols?
4:49 - Can you put BBs inside a ThunderB ?
6:35 - Why aren't there more movie replica airsoft guns?

9:30 - Code Red Headsets Video of the Week: BLASTING V. | SILVERBACK SRS GHILLIE SNIPER GAMEPLAY - Hoak07"