Hi Kate, the shoulder is not a stable joint, and this is so it can also be the most flexible joint in the body. It is also the most complex. If one is mounting the gun in the correct position, it will not impact on the long head of the bicipital tendon, the condition your hub had I believe you said, imping syn, but it will be quite close to the short head, which lies closer to the body midline.
I did meet one fellow who complained of shoulder px post shooting, and it only took 1 shot so see why. The gun didn't fit him so he had gotten into the habit of putting the gun on the bicep tendon just distal (down) from the ant. deltoid (front part of the shoulder muscle). This was cured when he bought a DT10 that fit him like a glove.
eiderz makes a good point with recoil and the neck (cervical region) and it is speculation on my part, that micro concussions may also be triggered by a small or weak person shooting a heavy recoiling gun repeatedly. It's also possible that many of the trap shooters had other problems that precipitated the neck px. I have not had any trap shooters as patients so this is speculation on my part.
Back to your shoulder... I am assuming you have had an MRI done and it did not show any bicipital impingement, but should have pin pointed the problem. I'm thinking it is more likely that you have either bursitis or small avulsions (tears) of the short head of the bicep or of one of several tendons comprising the shoulder girdle that give integrity to the joint. Or even a laboral tear is possible, but again the MRI should show this clearly.
Can't think of a single shooter that ever came into the clinic in 3 years, complaining of shoulder pain. I see and treat neck (cervical) problems all day long, but most are do to poor posture, trauma, disease, or IMO, sleeping on your stomach with your head severely turned to one side. (I call this face sleeping) This I believe causes inflammation and subsequent degeneration of the facet joints.
I do believe tennis players and golfers do have frequent shoulder and elbow problems. There are even syndromes named for those sports, tennis elbow and golfers elbow, depending on if it is the medial or lateral ligament and its epicondyle involved.
So, a betting man or betting lady would focus on the tennis as the original source of the problem, and the shooting as merely an aggravating factor.
I have to run, I'm off to the last couple of days of X-Ray review... Been 6 long days but it's almost over.
Best of luck to you, and wishing you a speedy recovery.
Jim