Ron,
Im stating this thinking your using a factory CVA plug you modified?? I would be very careful doing what your doing with that plug. Its obviously very weak material and its not designed to take all that pressure that your putting on it. That 5/32 flame channel or even 1/8 will hold powder in it. The reason your seeing higher velocity is because of more pressure building in the flame channel aside from the barrel. I honestly wouldnt do this with a Lehigh 17/4 plug with ventliner removed. Very dangerous!!!