I recently purchased an M4A3 and I sighted it in with the iron sights and I had to max the windage out on the rear aperature. I knew from my experience with them in the Army that something was awry, so I wrote them and this is what they emailed me back with:
"The barrel needs to be reindexed to the upper receiver to correct the windage. If you return the barreled upper to us we will correct that under warranty for you. It is best to call our Customer Service people at 1-800-883-6229 for an RMA return number to have that done.
Thank you,
Jim Eden, Tech Support "
Just thought I would post that if anyone was having similar troubles.
SAW