strange question
 

i have some new sheet metal that I want to plug weld in an area where there is no hole in the metal. I want to poke a small hole in one side to plug weld.
I have a spot weld cutter but its 5/16 kinda big... any ideas on what i can use?
I regualr rill bit will go through both so that's out. something 1/8 inch maybe?

I take it the panel is already installed? Can you get the the back side? If so you can place a piece of copper on the back side when you weld, just drill it out the size you want and then with the copper (or a welding paddle) you fill back in the hole. If you can't get the the back side I would go with the spot weld drill bit and just fill in the bigger hole. Or if you can get to the back side and have access to spot welder that would work the best so you just fuse the two pieces together.

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.

Use a die grinder and cut through the surface of one layer?