It's involved and will require tools but I think the 3mm peg should be cut off, a 5mm hole drilled in it's place and a new length of 5mm plastic rod glued in to make a new peg. And if you really want to make sure it doesn't pull back out, drill another hole in from the side through the gun and through the new peg. This locking hole doesn't have to be big, just enough to fit wire from a paper clip cut to length and glued never to be removed.

It may seem easier to build up a layer of putty or some kind of paint/lacquer around the existing peg but it's my experience that the extra layer just won't survive the wear and tear a weapon peg goes through.