What I do:
Use a countersink 45 or 60 degrees to break the edge of the hole, then install a screw a bit longer so it sticks out a few thousands, if the hole is bored through I use locktite or something simular to lock the screw. cut the head off and very carefully pein or rivet the top of the screw so it fills the top of the hole and the countersink bore. Then file off the remaining part of the screw flush with the receiver. The reason you need to use a counter sink is to get below the threads so you don't have a line where the thread starts, otherwise you will have a dark line part way around the screw.
If done correctly peining the screw will cause it to fill all the voids and with a countersink will blend in with the receiver, you will have to blue or refinish the receiver when done. You might want to see if the screws you use will blue to match the finish some don't take blue well.
It takes practice to get it right, you might wish to practice on a piece of round stock if its the first time to see what it takes.
I have filled a lot of holes like this and when finished you won't see the spot.
I prefer this method as I don't have a Tig welder