Went and looked at picture of a Gateway Drum Smoker, I see at least two large vents for intake and one large outlet chimney. First glance, looks like the smoker will work.
Charred taste is usually caused by creosote in the smoke, that points to a fire that is not burning cleanly. It can be as mentioned, an intake issue, of it can be from poor fire management else wise. Are you running the chimney wide open? It should be. Are you burning the fire long enough that the smoke is barely visible? It should be very light blue and see through. Are you letting the heat stabilize before putting on the meat, then letting it stabilize again before fussing with the vents? You should allow at least 30 minutes after putting on the meat, into a stable cooking environment, before fussing with the vents. (oops, simulpost)
At those temperatures, how much sugar is in your rub? A rub that is fine for ribs, will not be fine for a pork butt, at 325F. I have had no issues at up to 275F, but, there can be excessive darkening of brown sugar at 300F on a butt, that you do not see on ribs.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."