5/16 isn't ideal, but it's not overly huge, either. I usually use a 3/16 or 1/4" bit for plug welds, but there have been times where I've had to use the holes left behind from spot weld cutters as the hole for a new plug weld. Those holes are usually around 3/8", and just require a little more time and wire to fill. You may want to knock the amperage down a bit to avoid creating too much heat when filling such a big hole, but not too much or you won't get good penetration.
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