the sxs is definitely inertia and wasn't getting enough resistance to reset. Working with snap caps if I bang it on desk. The O/u is working fine with snap caps and must be mechanical it fires both perfectly.
While it may be mechanical, I would think this more than likely points to ammo being too light. You mention "gun" here, but your original post says two guns did the same thing. If they both work with snap caps, I think the problem is the ammo. A good cleaning never hurt a gun, but if the ammo is too light, it may never work in the gun. Same problem you see with semi-autos and low pressure rounds. One of the big reason you will never see mass market ammo with really low pressure loads.
