During the First World War, the standard issue rifles of the countries participating were bolt-action rifle, meaning the user had to manually pull back the bolt and bring it back into firing position to load the next cartridge. Generally, the rifles were low capacity compared to the rifles we have today but the ammunition loaded into the guns were also bigger, ranging from around 7.7mm-8mm cartridges. They were known as "battle rifles" because of the powerful cartridges they fired. Though after the Second World War, the cartridges were reduced in size to allow for troops to carry more ammunition.
During the First World War, bayonets were given to every soldier in the army because of the way war was foguht at the time. Having a bayonet allowed soldiers to have their rifles double as a spear. The bayonets fell out of use as the rifle became shorter and as militaries began using more firepower. Bayonets were still used in WW2 and Cold War wars too.
Rifles during the First World War used iron sights, but could be fitted with a scope. The grips of the rifles varied but some of them had a pistol grip such as the Springfield and Lee-Enfield.
The standard issue rifle changed a bit during the period between the wars. A really noticeable development during the war was the assault rifle. During the Second World War, Germany created the world's first successful assault rifle, the StG 44 or Sturmgewehr 44. Assault rifles are rifles that can fire automatically or in selective-fire. The assault rifles has a wider range of use than the rifles used before. It replaced SMGs and battle rifles and by the end of the 20th century were standard in most militaries around the world. Though, they weren't use that much during the Second World War with only one country, Germany, using them and even they didn't use them a lot.
Another change that occured with rifles was semi-automatic firing. Semi-automatic riles are rifles that automatically load the chamber and eject the used cartridge after a bullet is fired.The best known were the American M1 Garand and German Gewehr 43. The Russians used one too but their rifle's production was halted because of attacks on their factories and forcing them to return to bolt-action rifles.
Today, most of the modern standard issue rifles are assault rifles. The rifles used today are mostly the same as the rifles that were developed during the Cold War but come with better parts and attatchments such as better sights, forward grips, etc. Also, bayonets aren't used much today as they were before. Based on the combat footage I've seen online, the engagments are mostly in long range and the combatents don't get within melee range.
A wide variety of handguns were used during the First and Second World Wars and were issued to officers, cavalrymen, tankers, or anywhere a rifle would be impractical. The most well-known being the German Luger and American M1911. The handguns carried by the soldiers in World War 1 included both revolvers and pistols but pistols were favored over revolvers by militaries later on. The M1911 was one of the most used pistols by the end of the Second World War and was used by a lot of the Allied militaries. Though, they have not changed much since then. The M1911 and a few others guns are still used by militaries today.
During the First World War, modern flamethrowers first introduced. They used a flammable liquid and were used to attack fortifications such as trenches, bunkers and pillboxes. They were also used as weapons of phycological warfare and caused horrible and painful deaths for the victim. They were replaced by rocket launchers because of their relativiely short range compared to other weapons and heavy weight. The last US flamethrower was used during the Vietnam War.