Just another wild idea, similar to what was mentioned above, if it’s just a buffer tube, and could be trashed anyway, I would probably try padding it with something like duct tape or electrical tape, and putting some good grips on it, like vice grips or channel locks, heating that end cap thingy up quite hot with a torch. Maybe even really (red) hot, as long as you don’t think too much heat is transferring to the upper receiver. Try to unscrew it or break it free with a Nother set of channel locks while it is hot and as you are heating it periodically.
You have to be ready to ruin it. But I would think there’s a solid 70% chance you get it off.
You have to be ready to ruin it. But I would think there’s a solid 70% chance you get it off.