"and the booby-trapped 105 round blew him into a tree."

A 105mm round is typically used in the Howitzer, a large cannon used to destroy enemy bases or vehicles from several miles away. But a 105mm round can also be used as the explosive in an improvised munition, detonated remotely.

The United States expended more bombs and artillery shells in Vietnam than in World War II. By contrast the Viet Cong used guerilla warfare tactics, particularly booby traps. Unexploded artillery rounds or bombs could be refashioned as land mines. These caused severe injuries such as loss of limbs or death amongst US infantry personnel.

Below is one of 26 Private SNAFU ('Situation Normal, All Fouled Up') cartoons made by the US Army Signal Corps to educate and boost the morale of the troops. Originally created by Theodore Geisel (Dr. Seuss) and Phil Eastman, most of the cartoons were produced by Warner Brothers Animation Studios:

 

 

Many other concealed devices were deployed against Americans during the Vietnam war:

Panji traps - Long nails or lengths of thin steel rods, hammered flat at the ends, filed into a barbed shape, then hammered through blocks of wood.

Cartridge traps - A section of pipe or bamboo with a tack nailed through the base upon which sits a cartridge with the tip slightly above ground level.  When a soldier stepped on the cartridge, the pressure caused the tack to ignite the primer, firing the round up through his foot.

Spike Board - A treadle board, one end of which is spiked. When a man steps on the treadle, the spiked end flies up striking him in the face or chest.