The P-38 is known as a "John Wayne" by the United States Marine Corps, because of its toughness and dependability. Another theory for the "John Wayne" name is he used one in the army training film for the can opener to open a can.  The can opener is pocket-sized, approximately 1.5 inches (38 mm) long, and consists of a short metal blade that serves as a handle, with a small, hinged metal tooth that folds out to pierce the can lid. A notch just under the hinge point keeps the opener hooked around the rim of the can as the device is "walked" around to cut the lid out (see photo).  

Official military designations for the P-38 include "U.S. ARMY POCKET CAN OPENER" and "OPENER, CAN, HAND, FOLDING, TYPE I". As with some other military terms (e.g., "jeep"), the origin of the term is not known with certainty. The P-38 opener coincidentally shares a designation with the Lockheed P-38 Lightning fighter plane, which could allude to its fast performance.  One explanation for the origin of the name is that the P-38 is approximately 38 millimeters long. However, use of the metric system in the U.S. was not widespread at this point, and United States Army sources indicate that the origin of the name is rooted in the 38 punctures around the circumference of a C-ration can required for opening. P-38s are no longer used for individual rations by the U.S. military, as canned C-rations were replaced by MRE rations in the 1980s, packed in plastic pouches.  

The original U.S.-contract P-38 can openers were manufactured by J.W. Speaker Corporation of Germantown, Wisconsin (stamped "Speaker USA") and by Washburn Corporation (marked "US Androck"); they were later made by Mallin Shelby Hardware inc (defunct 1973) of Shelby, Ohio and were variously stamped "US Mallin Shelby O." or "U.S. Shelby Co."  The US Shelby Co. version is the main one issued by the military.

The P-38 is easily used. First, the cutting point is pivoted to its 95-degree position,^[5] from its stowed, folded position. Then, for a right-handed user, the P-38 is held in the right hand by the flat long section, with the cutting point pointing downward and away from the user, while also hooking the edge of the can through the circular notch located on the flat long section next to the cutting edge. The can is held in the left hand, and the right hand is rotated slightly clockwise, causing the can lid to be punctured.

The can is then rotated counter-clockwise in the left hand, while the right hand rotates alternatively slightly counter-clockwise and slightly clockwise, until the can has been rotated nearly 360 degrees and the lid is nearly free. The lid of the now opened can is lifted, most often with the P-38 cutting edge, and the P-38 is wiped clean, and the cutting point is rotated back to its stowed, folded position. The P-38 is then returned to its stored location, whether that is dangling on a dog tag chain around one's neck, or in one's pocket if the P-38 is attached to a key ring.

Left-handed users simply hold the P-38 in their left hand, with the cutting point aimed towards themselves, while holding the can to be opened in their right hand, while also reversing the sense of the cutting hand movements just described. By tradition, 38 cuts as just described were supposedly required to open a can of C-Rations.