jammed spring retainer of a Winchester 1300 shotgun
Thanks for your input, which suggested that something else might be wrong.
So I removed the magazine cap and found that the magazine spring retainer (which inserts into the magazine tube before the magazine cap is screwed over it) was sticking out of the magazine tube by about 1/4". Now I'm trying to remove and re-insert the spring retainer so that its surface is exactly flush with the threaded end of the magazine tube. That would allow the magazine cap to be screwed fully in untilit bottoms against the threaded end of the magazine tube, which would cause the barrel to fully seat and allow the trigger to be clicked.
PS: The gun was serviced by an authorized dealer 3 years ago, with little use in between. I'd never even removed the magazine cap (which conceals the problem of the non-flush spring retainer).