I seen pretty much the same thing Tuesday night. Guy walked up to the door of the club with the gun on his shoulder. He turned to talk to someone behind, and someone came out of the clubhouse swinging the door open and knocked the gun off his shoulder. Three guys reacted to try to stop the gun from hitting the ground. Once I seen it leave his shoulder, I closed my eyes and plugged my ears. Needless to say his gun has some bad dents from the blacktop.
The thing that has to be done is to raise the grain as much as possible so it fills the dent. Depending how big and deep the dents, you may have to remove the finish and start over. Then the stock needs to be sanded and finished. Like BigM-Perazzi says it takes steam and heat to raise the grain, or expand the wood. If you can't get the wood to expand fully then you have to fill the dent. I use either clear epoxy or some built up finish. Sand it smooth to the surface, like doing bodywork on a car. Then refinish the hole stock. If you are going to stain the stock remember to do this under the fill first. Good luck, or have it done by someone who knows what they are doing. Jon