Yeah, the M26 Dart firing assault rifle bullets was patched and it's now back to firing lame shotgun pellets. Battlefield 3 patch notes can be found in the obvious place, the Battlefield blog.
You can find the patch notes for the specific patch that fixed the glitch by Googling "June 4 5 patch notes" or just "Battlefield 3 patch notes". There was more patched than just the M26 dart, so reading the patch notes might be useful to find what glitches you still can do.
Good luck in Battlefield. :P