Gordon Ingram served in World War II as a young man. When he returned to the USA after the war ended, he had considerable knowledge of submachine guns and firearms in general. His belief was that simple designs made for the most reliable weapons, and that automatic fire was essential for modern weapons. Using this knowledge, he created his own submachine gun, which he named the Model 5.
Only one prototype was made; it was a very basic submachine gun with only three moving parts - the trigger, sear and bolt. The barrel housing was made from perforated seamless tubing. A non-detachable wooden buttstock was fitted, but still the weapon was very light at only 6lb. This prototype was made in 1946 and it appeared in a sales brochure from the Lightning Arms Co., where it was advertised as the "Lightning Model 5". Despite this, there are no records to suggest that it was ever sold commercially or developed past the sole prototype model. 1946 was a very poor year for weapons sales and launching a new design was difficult since there was plenty of war surplus in circulation. Allegedly, the Nicaraguan government was interested in the Model 5 and Ingram actually took the prototype to Nicaragua for a demonstration, to which orders may have been placed, but for whatever reason the Lightning Arms Co. did not produce the weapon. I cannot confirm this story although I have no reason to doubt it. In 1947, Ingram began work on his Model 6, which achieved considerably more success, and later his Model 10, which became one of the world's most famous submachine guns.
Although the naming of the Model 5 suggests that this was Ingram's fifth design, he actually chose to call it the Model 5 to avoid confusion with the US Army's M1, M2, and M3, and to allow for a possible M4.