Took my 870 pump 12ga out today to do some squirrel hunting and to my surprise found that after each shot the bolt had moved back a quarter to half and inch by itself and was exposing the shell brass. This is before I attemted to eject the spent casing. It also looks like some damage has been done to the fingernail shaped cut out on the barrel "flange" that goes into the receiver. I'm pretty sure that this is not normal. Any suggestions/thoughts? The gun was purchased last fall brand new and has less than 20 shots through it.