In the 1950s and 1960s, Interarmco flooded the American market with military firearms, catering to souvenir-hungry GIs and sportsmen, and drastically undercutting domestic sporting-arms manufacturers. At the same time, Cummings became an export sales agent for various small arms manufacturers. Interarmco was an original exclusive agent for ArmaLite, and Cummings personally demonstrated its revolutionary AR-10 selective-fire rifle to various nations, including Nicaragua and the Dominican Republic. The Nicaraguan demonstration succeeded in what would have been the first-ever AR-10 export sale to the nation of Nicaragua in 1957 (later cancelled). In 1958, Cummings sold 100 ArmaLite AR-10 rifles to Cuban dictator Fulgencio Batista, but in 1959 the entire shipment of AR-10 rifles was captured intact on the Havana docks by the victorious rebel forces of Fidel Castro. Cummings wrote Castro and asked him if he would pay for the rifles or return them, and was invited to visit Cuba in return. Reportedly impressed with the firepower of the AR-10, Castro paid for the rifles and asked for more, but the American arms embargo to Cuba prohibited further sales. Castro later gave the rifles to rebels seeking to overthrow the leader of the Dominican Republic, General Rafael Trujillo. In June 1959, the rebels invaded the Dominican Republic in a combined air/sea operation. Betrayed by local residents, the seaborne rebel forces (led by Cuban officers), were surprised at the water's edge; those dropping via parachute were hunted down in the following days by the Dominican army. Captured AR-10 rifles from Cummings' Batista shipment were found on the bodies of guerrillas killed in firefights with government forces. When Cummings arrived in the Dominican Republic that same month to discuss the subject of arms sales with the nation's arms procurement officer, an enraged General Trujillo stormed into the room carrying an AR-10 rifle taken off the body of a dead insurgent, demanding to know why Cummings had supplied guns to his enemies. During this time period, he became a British subject and moved to Monte Carlo, Monaco, while maintaining warehouses worldwide and the company Headquarters in Alexandria, Virginia.
