I have had issues with pain and inflammation in the AC joint on my right side. These issues were already present when I resumed barbell training approximately a year and a half ago, and strength training has made my shoulders feel better overall. Never diagnosed, but I think I separated my shoulder about 20 years ago playing high school football. I had a fair bit of trouble with my shoulder at that time, including some stingers. The trainer didn't seem to know anything and I never went to see a doctor. I do have a visible bump and that's exactly where it hurts when things flare up.
The AC joint really only gives me trouble on "heavy" bench sets, particularly on reps 4 and 5. I can get the bar off my chest then about halfway to lockout the shoulder starts to feel unstable and the pain inhibits me from applying full force to the bar. Pushing through the pain results in increased inflammation that lasts for a few days. Heavy triples and light/medium sets of five generally don't give me a problem. I do shoulder dislocates with a resistance band to warmup before squats and bench. I've experimented with wider and narrow grips and have settled on ring fingers on the powerlifting rings on the barbell.
Would appreciate some feedback on my bench press form.
Bench 262.5x3x2 - YouTube
Bench 237.5x5 - YouTube
Form is not a problem. But you do need a shoulder MRI.