I was in my first 3 gun match this weekend and had problems with my nearly new Remington 870 Marine Magnum 12g. I was shooting 2 3/4" bird shot and later slugs. The action nearly always jammed locking the slide closed. With much effort of forcing it a little forward then pulling back hard it would cycle. This added a lot to my time and was very tiring. Another competitor offered me his gun it was so bad. Any suggestions?