The original design of the M1 Garand as adopted in 1936 used a “gas trap” system instead of a gas port drilled in the barrel. This system used a type of muzzle cap and false muzzle to redirect gas into the gas cylinder in the short distance between the end of the rifled barrel and when the bullet left the muzzle. The system worked, but was not ideal.
Several problems were found with the gas trap system as the guns went into production. These included cleaning complexity, an unstable front sight, and a potentially weak bayonet mounting point. Most significantly, however, one rifle in testing had a screw work loose in the muzzle cap, which allowed parts to shift out of alignment and resulted in a bullet striking the gas plug and blowing the entire assembly off the gun. This led to a decision to redesign the gas system of the M1 to use a simpler gas port drilled in the barrel.
When this design change was made, 18,000 rifles had been completed and parts were made for an additional 33,000. Those guns were completed with the available parts, and the new gas system was used for all further production. Gas trap M1s are very rare today because the guns were updated to the new system when they were overhauled during WWII, and in 1947 the Army ordered all remaining gas trap rifles destroyed.