Tig
Have them TIG welded and then file and sand the weld down to the barrel or reciever. Not only is the best way, but if you're going to reblue or park, it's the only way. if you're just going to use a spray finish, get some metal paste such as lab metal and use it with a spatula and overfill the pit and then sand the patch back to the barrel or reciever. There are other metal paste but the lab metal works very well. Good luck,