When I was in the Army I took part in a lot of shoooting competitions. I was at the firing line and only discovered after my first shot that I had forgotten to bring my ear plugs with me.
If I would have stood up or left the line I would have been disqualified for that phase of the competition.
I shot 21 rounds of 5.56 and my ears were ringing and kept ringing for hours. After that when people spoke it sounded like they were underwater.
A year later when I took my normal Army physical I had hearing loss in the high frequency range.
I wouldn't advise training without hearing protection to try to inure yourself to the blast, it's just not worth it.