Sorry I can't provide any proven low pressure 20 ga. loads, but I'm sure the guys here will help you. I'm wondering how thick damascus barrels in a 20 ga. Flues are. I have four 20 ga. Flues that are fluid steel, and two are parts guns due to blown barrels. Both let loose just ahead of the forend and the walls are quite thin there. I know nothing about the loads that popped them or whether bore obstructions were involved. Because of those thin walls, dents may have been a factor. I recently found and fitted a set of barrels to a 16 ga. Flues that was also blown in the same area, and also very thin. On the flip side, I witnessed a friend (many times) fire factory 3" magnum loads in his dads' Flues 20, and he got away with it. I'm sure this did nothing to extend the life of this gun.
