This is called phoria and I believe it is discussed in the large "Best General Purpose Optic" thread tacked above. It will also be covered towards the end of your ACOG mannual. Phoria is worse for some people than others and it discusses why very briefly in the owners mannual. Several inches is much much more than I have noticed? How far away is the target you are looking at when you do this test?
BAC is only for close quarters shooting. The advanage of BAC is that you have the ability to use a magnified optic like a dot site when you are cruising around in fluid movement type fast paced shooting environments. The only time you would really be utilizing BAC is at fairly close ranges.
Therefore, when you check to see how much of a difference phoria makes for you, test it on targets, I dont know... say about 10 yards away or so? See what the differences are at ranges like that. For me is is only a matter of a couple inches at most. A couple inches usually doesnt amount to a whole lot of anything in fast paced shooting like CQB.
If things are so tight that an inch or two is going to be crucial, dont use BAC is possible. Just take whatever time may be available and make the well aimed, precise shot using the magnified image. It shouldnt take you longer to do that than it should to get a clear sight picture/sight allignments with irons.