Not to belabor a point and I certainly agree with Tom and all about safety. To clarify my suggestion, I figure if you have drilled a 3/8 inch hole you would have to go up to a 7/16 inch tap, not redrill a 7/16 inch hole. tap with the appropriate tap for a 3/8 inch hole and make your own vent was what I meant to say.
If it is all that confusing, you are smart to advertise for someone to fix it for you. Still it is a learning experience. Pay someone to do it or purchase the tools you need and learn to do it yourself. Either way you get to contribute to the economy. In the grand scheme of things it will not be your worst mistake. I can one up you on this many times over. I've drilled the vent hole through both sides of the barrel. Beat that one. I've cut barrel tennon dovetail slots into the bore. These things can get expensive but I've never done them twice and hopefully as in this case you can recover from them. Just err on the side of safety.
