Good Afternoon yall. I have a slight hiccup in my process of restoring my grandfathers 1950 Mossberg 151M-B. The gun was never properly maintained before the gun was pasted on to me and I am trying to correct 70 years of bad maintenance. I have tried removing the breech plug to clean the internals and nothing worked. I let it sit in oil for 24 hours and then tried to remove with Channel locks and a flat head screw driver. What should I do next. Any advice helps. I will most likely have to order a new breech plug once I get the old one out. The photo was taken before I let it sit in oil

What kind of oil? Soak it in kroil and be generous with the oil. And use a vise with soft jaws or at least a piece of leather wrapped around it, gripping guns with channel locks is counter productive to firearms restoration, and good luck if you shear it off.....