When drilling for under lugs, particularly on a pre-shaped stock.
Do your measurements, and mark both sides of the stock. I lay a small machinist's square across the top of the stock to make sure both sides are marked equally, and then continue the lines down the sides using the square.
Then use a hand drill, I prefer an old egg beater type, and make small guide holes, just enough to contain the bit in drilling.
Use a vertical centering pin in your drill press vise, and drill half way from each side.
Do not have the barrel in the stock to do this!
Take the stock back to the bench, and use the hand drill again, running it through the holes to assure alignment.
Clamp the barrel into place in the inlet, and once again, use the hand drill to lightly mark the alignment on the lug. Remove barrel, and examine if your placement is correct. If too high or low, plug the holes, and redrill.
Then clamp the barrel into place in the stock, and drill the lugs.