IMHO a bad idea.
In a real life situation they say you will not hear the shot as you would at the range.
Since I have never been in combat I will have to take others word on that, but while big game hunting my ears have never rang or been bothered by the sound of a 30-06.
Also I have heard pistol shots on the streets from time to time and they sound more like pop-pop-pop than loud booms.
So, to train without hearing protection for so called realism is, IMHO, just asking for health problems down the road.