When you shoot a gun with one of these games, the gameboard draws a frame of white screen to determine where on the screen the gun is pointed. This frame of white can tax the monitors B+ circuit and minor changes in screen geometry can occur as B+ changes for that brief period. It can be common, and normal, on some monitors due to the design of the B+ regulator.
If the geometry change is more than minor, it might indicate poor B+ regulation due to a problem in the circuit. Capping the monitor and touching up solder joints might be a good place to start.
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