Perhaps a cannon was considered an elephant gun at the time?

This Maharajah's Shooting brake or car came equipped with rifles,shotgun, "Howdah" pistol, a towed machine gun and a cannon!

A Rolls Royce Hunting Car - ex-Maharaja of Kotah "Tiger Car"

1925 Rolls-Royce New Phantom Torpedo Sports Tourer

The Tiger Car as presented tows a rare carriage-mounted Bira .450 caliber hand-cranked machine gun, once considered suitable for hunting Bengal tigers. Onboard firearms include a 'Howdah' gun—a large-caliber, short-range firearm useful for repelling beasts bent on attacking the car's occupants. Two lockable vertical racks within the interior contain an assortment of vintage big game rifles, bird guns and shotguns. The Lantaka cannon mounted to the rear bumper serves as an elephant gun.

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