The Lahti L-39 is a Finnish 20 mm anti-tank rifle used during the Second World War. It had excellent accuracy, penetration and range, but its size made transportation difficult. It was nicknamed "Norsupyssy" ("Elephant Gun"), and as tanks developed armour too thick for the Lahti to penetrate its uses switched to long range sniping, tank harassment and with the L-39/44 fully automatic variant, employment as an improvised anti-aircraft weapon.

Aimo Lahti had doubts about the original idea of a 13 mm anti-tank machine gun and started working on a 20 mm design. Officers who wanted smaller calibre anti-tank weapons believed that the muzzle velocity of 20 mm shells was insufficient to penetrate armour and a weapon with a higher rate of fire and in a smaller calibre would prove useful. As a result, Lahti designed two competing anti-tank weapons: a 13.2 mm machine gun and a 20 mm rifle. After test firing both weapons in 1939, they found that the 20 mm rifle displayed better penetration.

The rifle is a semi-automatic, gas operated weapon with the piston located beneath the barrel and ammunition feed from detachable top-mounted magazine with bottom ejection for the spent cartridges. To reduce recoil, the rifle is equipped with a five hole muzzle brake and a padded leather recoil pad. The barrel had a wooden jacket as to allow transportation after firing.

During the Winter War (1939–1940) Finland lacked anti-tank weaponry. Only two 20 mm rifles and a few 13.2 mm machine guns made it to the front, where the 13.2 mm machine guns were found to be ineffective and unreliable while the larger 20 mm rifles proved successful against Soviet armour. Because of this, Finland finally settled on the 20 mm design and started production.

The gun was also widely used in the Cold Charlie counter-sniper technique, where the Finns would use a mannequin posing as an officer sloppily covering himself. Soviet snipers would fire upon the mannequin, and the Finns would then return fire at the Soviet snipers with the Lahti L-39.

Several of the rifles remained in service after World War II serving as an anti-helicopter weapon, while many others were sold to collectors, mostly in the United States. Today the rifles, especially those in working condition, are quite rare and highly sought after. Some deactivated weapons (with a steel bar welded into the chamber) have been reactivated due to their value. Ammunition is rare. Often they are rechambered to .50 BMG to lower the cost of use. In the United States of America, civilian ownership remains possible, depending on state and federal laws. Because the weapon fires rounds larger than .50 calibre, it is considered a destructive device and is subject to the 1934 National Firearms Act. Civilian ownership is dependent on compliance with this law and whether one's state law prohibits civilian ownership of destructive devices.

Users found the L-39 to be heavy and difficult to move in the battlefield. Even its magazine weighed almost two kilograms. The magazines had a covered viewing slit on the right side to indicate the number of rounds left in the magazine, and a 15-round magazine was later developed for anti-aircraft use.

The whole weapon weighed some 50 kilograms and it was usually towed by horses, but when stripped down could be carried by several men. The rifle had adjustable iron sights calibrated between 200 and 1,400 meters and was equipped with unusual dual bipod, with two sets of legs, one with spikes for use on hard ground and the other with skids for use on softer ground or snow.

In the field, a two-man team was assigned to the gun to move and fire it. Some rifles were abandoned in the heat of battle, but they were easy to replace. By the end of the war over 1900 L39s had been manufactured by VKT (Valtion Kivääritehdas, "State Rifle Factory", modern day Patria) and put in the field.