It was metal. Even wooden planes were still killed by cannon rounds.
You sure it wasn't just lag? There was a bug/issue during the beta where the I-16 absorbed gobs of ammo. A squaddie and I were dueling in them and often ran out of ammo (hitting each other) with neither one getting a kill. This was using non-cannon loadouts, mind you, and it was fixed later before its release. Best way is to take a pal up in the DA or empty EWA and test it out.