The technical name for the "Deck Gun" is the "Stang Inteligiant Nozzle" as per the OLD FDNY All Units Circular about the Super Pumper System.
The large Stang Inteligiant Nozzle on the cab of the Super Pumper Tender was rarely used. When it was used, often the S.P. Tender Cab was disconnected from the trailer, the trailer was left back and the much easier to positioned cab was move up to the point of operation. The only time I saw this done was at a few Pier fires and the Tender's "Stang" =Inteligiant Nozzle was being supplied by a Marine Co, Fireboat. Most Commonly the Super Pumper Tender was used as a very large Hose Wagon, and it carried about 1/4 to 1/2 Mile of 4 1/2" hose. The Tender was used also as a large Hose Manifold unit, to help supply other units. Captain Bob Rainey FDNY Engine 26 retired.