AN AR, I'm guessing?
A very typical error that folks new to reloading make-- as mentioned above--not setting back the shoulders far enough. You don't really need small base dies or gages, just follow the directions for setting up the dies exactly. In my experience rounds in which the bullet has been seated too far out to chamber, will not fit in the magazine of an AR and/or will be reseated when the bolt closes as long as the bolt is allowed to close with full spring pressure. Not the best way to seat an accurate round.
Here is a useful essay on handloading for gas operated rifles. It comes from Sierra's manual, something worth having.
http://www.exteriorballistics.com/re...sgunreload.cfm