Absolutely! After several years most necked factory ammo becomes brittle. In most cases the bottle necked brass was annealed at the factory and because of that becomes brittle over time. In double rifles there will always be microscopic difference in the chambers. If the chamber in the bottom barrel is slightly looser in the throat area it may be just enough to let the neck expand slightly. That will cause the cracks like the ones you are experiencing.

Just buy some new ammo and I suspect there will be no cracks from the bottom barrel.
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