Several posters complained about the M-14 steel butt plate. Never a problem for me, and I'd flip up the butt plate hinged cover for prone firing to keep the rifle placement really consistent.
The only thing that got sore for me was my cheekbone, right under my right eye, from my spotwelded thumb knuckle if we had a lengthy range session.
IIRC, some of those were more than 100 rounds. I remember one rapid fire/reload field of fire exercise burned off 100 rounds rapid fire. Some of the guys made the mistake of touching or grabbing the barrel afterward.
Bad idea. They only did that once.

We didn't know what earplugs were, so I think I lost some hearing, but at the time, the noise didn't seem bad unless somebody fired too close on the right. In that regard, long barrels are a Good Thing.