Trench warfare
The Germans used the Wurfmaschine, a spring-powered device for throwing a hand grenade about 200 m (220 yd). The French responded with the Sauterelle and the British with the Leach Trench Catapult and West Spring Gun which had varying degrees of success and accuracy. By 1916, catapult weapons were largely replaced by rifle grenades and mortars.
The Germans employed Flammenwerfer (flamethrowers) during the war for the first time against the French on 25 June 1915, then against the British 30 July in Hooge. The technology was in its infancy, and use was not very common until the end of 1917 when portability and reliability were improved. It was used in more than 300 documented battles. By 1918, it became a weapon of choice for Stoßtruppen (stormtroopers) with a team of six Pioniere (combat engineers) per squad.
Used by American soldiers in the Western front, the pump action shotguns was a formidable weapon in short range combat, enough so that Germany lodged a formal protest against their use on 14 September 1918, stating "every prisoner found to have in his possession such guns or ammunition belonging thereto forfeits his life", though this threat was apparently never carried out. The U.S. military began to issue models specially modified for combat, called "trench guns", with shorter barrels, higher capacity magazines, no choke, and often heat shields around the barrel, as well as lugs for the M1917 bayonet. Anzac and some British soldiers were also known to use sawn-off shotguns in trench raids, because of their portability, effectiveness at close range, and ease of use in the confines of a trench. This practice was not officially sanctioned, and the shotguns used were invariably modified sporting guns.

The Germans embraced the machine gun from the outset—in 1904, sixteen units were equipped with the 'Maschinengewehr'—and the machine gun crews were the elite infantry units; these units were attached to Jaeger (light infantry) battalions. By 1914, British infantry units were armed with two Vickers machine guns per battalion; the Germans had six per battalion, and the Russians eight. It would not be until 1917 that every infantry unit of the American forces carried at least one machine gun. After 1915, the Maschinengewehr 08 was the standard issue German machine gun; its number "08/15" entered the German language as idiomatic for "dead plain". At Gallipoli and in Palestine the Turks provided the infantry, but it was usually Germans who manned the machine guns.
The British High Command were less enthusiastic about machine guns, supposedly considering the weapon too "unsporting" and encouraging defensive fighting; and they lagged behind the Germans in adopting it. Field Marshal Sir Douglas Haig is quoted as saying in 1915, "The machine gun is a much overrated weapon; two per battalion is more than sufficient". The defensive firepower of the machine gun was exemplified during the first day of the Battle of the Somme when 60,000 British soldiers were rendered casualties, "the great majority lost under withering machine gun fire". In 1915 the Machine Gun Corps was formed to train and provide sufficient heavy machine gun teams.
It was the Canadians that made the best practice, pioneering area denial and indirect fire (soon adopted by all Allied armies) under the guidance of former French Army Reserve officer Major General Raymond Brutinel. Minutes before the attack on Vimy Ridge the Canadians thickened the artillery barrage by aiming machine guns indirectly to deliver plunging fire on the Germans. They also significantly increased the number of machine guns per battalion. To match demand, production of the Vickers machine gun was contracted to firms in the United States. By 1917, every company in the British forces were also equipped with four Lewis light machine guns, which significantly enhanced their firepower.
The heavy machine gun was a specialist weapon, and in a static trench system was employed in a scientific manner, with carefully calculated fields of fire, so that at a moment's notice an accurate burst could be fired at the enemy's parapet or a break in the wire. Equally it could be used as light artillery in bombarding distant trenches. Heavy machine guns required teams of up to eight men to move them, maintain them, and keep them supplied with ammunition. This made them impractical for offensive manoeuvres, contributing to the stalemate on the Western Front.

One machine gun nest was theoretically able to mow down hundreds of enemies charging in the open through no man's land.
However, while WWI machine guns were able to shoot hundreds of rounds per minute in theory, they were still prone to overheating and jamming, which often necessitated firing in short bursts. However, their potential was increased significantly when emplaced behind multiple lines of barbed wire to slow any advancing enemy.
In 1917 and 1918, new types of weapons were fielded. They changed the face of warfare tactics and were later employed during World War II.
The French introduced the CSRG 1915 Chauchat during Spring 1916 around the concept of "walking fire", employed in 1918 when 250,000 weapons were fielded. More than 80,000 of the best shooters received the semi-automatic RSC 1917 rifle, allowing them to rapid fire at waves of attacking soldiers. Firing ports were installed in the newly arrived Renault FT tanks.
The French Army fielded a ground version of the Hotchkiss Canon de 37 mm used by the French Navy. It was primarily used to destroy German machine gun nests and concrete reinforced pillboxes with high explosive and armour-piercing rounds.
A new type of machine gun was introduced in 1916. Initially an aircraft weapon, the Bergmann LMG 15 was modified for ground use, with the later dedicated ground version being the LMG 15 n. A. It was used as an infantry weapon on all European and Middle Eastern fronts until the end of World War I. It later inspired the MG 30 and the MG 34 as well as the concept of the general-purpose machine gun.
What became known as the submachine gun had its genesis in World War I, developed around the concepts of infiltration and fire and movement, specifically to clear trenches of enemy soldiers when engagements were unlikely to occur beyond a range of a few feet. The MP 18 was the first practical submachine gun used in combat. It was fielded in 1918 by the German Army as the primary weapon of the stormtroopers – assault groups that specialised in trench combat. Around the same time, the Italians had developed the Beretta M1918 submachine gun, based on a design from earlier in the war.

Artillery: Artillery dominated the battlefields of trench warfare. An infantry attack was rarely successful if it advanced beyond the range of its supporting artillery. In addition to bombarding the enemy infantry in the trenches, the artillery could be used to precede infantry advances with a creeping barrage, or engage in counter-battery duels to try to destroy the enemy's guns. Artillery mainly fired fragmentation, high-explosive, shrapnel or, later in the war, gas shells. The British experimented with firing thermite incendiary shells, to set trees and ruins alight. However, all armies experienced shell shortages during the first year or two of World War I, due to underestimating their usage in intensive combat. This knowledge had been gained by the combatant nations in the Russo-Japanese War, when daily artillery fire consumed ten times more than daily factory output, but had not been applied.
Artillery pieces were of two types: infantry support guns and howitzers. Guns fired high-velocity shells over a flat trajectory and were often used to deliver fragmentation and to cut barbed wire. Howitzers lofted the shell over a high trajectory so it plunged into the ground. The largest calibers were usually howitzers. The German 420 mm (17 in) howitzer weighed 20 tons and could fire a one-ton shell over 10 km (6.2 mi). A critical feature of period artillery pieces was the hydraulic recoil mechanism, which meant the gun did not need to be re-aimed after each shot, permitting a tremendous increase in rate of fire.
Initially each gun would need to register its aim on a known target, in view of an observer, in order to fire with precision during a battle. The process of gun registration would often alert the enemy an attack was being planned. Towards the end of 1917, artillery techniques were developed enabling fire to be delivered accurately without registration on the battlefield—the gun registration was done behind the lines then the pre-registered guns were brought up to the front for a surprise attack.

Mortars, which lobbed a shell in a high arc over a relatively short distance, were widely used in trench fighting for harassing the forward trenches, for cutting wire in preparation for a raid or attack, and for destroying dugouts, saps and other entrenchments. In 1914, the British fired a total of 545 mortar shells; in 1916, they fired over 6,500,000. Similarly, howitzers, which fire on a more direct arc than mortars, raised in number from over 1,000 shells in 1914, to over 4,500,000 in 1916.