Action Screw:

If you can get three good threads to turn into the action, it should be enough. If not, get another screw from the hardware store and cut and/or grind to length. It must be short enough to not interfere with the closing of the bolt, etc. Most of mine are not much longer than that. You will know, though, when you go to seat it into the action. It will either hold good or it won't.