I'm looking for an accurate AS3 grammar (format in not an issue, but I presume ANTLR will feature the most) to use for a practice grammar I'm making.
What is the most accurate grammar available for AS3?
I think this one is pretty accurate if you are looking for an ANTLR grammar: AS3.g
This grammar has been originally developed a couple of years ago by Martin Schnable and then extended for the Meta-AS project. There are of course other ActionScript 3 parsers available as well, but they do not use ANTLR or another compiler-compiler.
So this is probably your best choice :)