According to this publication , Microsoft provides the MSIL grammar in the Microsoft Visual Studio installation file asmparse.grammar. If you do not have MSVS, the grammar is also indicated in the publication (see page 79).
If I am not mistaken, this grammar could simply be copied and pasted into the ANTLR grammar file (i.e. LL grammar, not LR one). If you are not familiar with the difference between the two, check out this ANTLR article .