I ran into a problem on a GM drop in barrel that I didn't like, the flash channel was only drilled part way under the nipple.
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When I brought this up, everyone with a similar barrel said this was standard for a GM barrel and it worked just fine.
I wasn't fine with me, I used a Dremel and a round diamond bit to remove all of the metal you see blocking the fire from the nipple to the charge, I cut the excess at a 45-degree angle to direct the flash toward the main charge.
One of the nay-sayers was adamant that I would damage the nipple threads with my Dremel. I have years of experience using a Dremel as a power decoy carver as well as collection of bits to do almost any job that comes up, the only metal I cut was the excess in the bottom of the hole.
Do you have any excess metal in your flash hole under the nipple?