There are few hard-and-fast rules, although, for the sake of relative completeness, you have to look at each participant in turn, then look at their first-line rifle, their reserve rifle and then whatever they scrounged up.
USA, for example, had 2 factories, both making the 1903 Springfield..... but they actually used almost 3 times as many Enfield M-1917 rifles in combat as they did Springfields. They also used Springfields right back to the 1873 as training and guard rifles.
Italy had their official Fucile 91, the original Carcano long rifle...... but they tubed hundreds of thousands of old Vetterli-Vitalis because they had to have something to shoot... especially after the disaster at Caporetto.
Japan was mostly on Japan's side, doing some Pacific patrolling and keeping their eyes firmly fixed on the German naval installations in China..... which Japan ended up owning, along with the Marshall and the Karoline Island chains. They sold most of their old-model Arisakas to Britain for training rifles, along with a few Type 38s, cleaning out their arsenals and making a buck at the same time.
Austro-Hungarian Empire officially used the 1895 Mannlicher straight-pull, but they also used huge numbers of their older models, the 1888, 1888/90 and 1890 because they needed enough guns for the war. Losses, especially in the "Bergerkrieg" often were shocking; in the end, more than 90% of all Austro-Hungarian soldiers were on the casualty list at some time.
Bulgaria was a major power in the Balkans, used the same equipment as Austria-Hungary.
Portugal was only a small country but it tried hard and performed out of all proportion to what could have been expected. They had an official rifle and ammunition of their own, the 1904 Mauser-Vergueiro in 6.5x58. They were (and are) Britain's oldest ally, going back even before Henry the Navigator, but Portugal was and is not a rich country. Their German-made rifles could not be replaced, nor could the German-made ammunition, so they ended up using mainly British equipment. The blood, however, was pure Portuguese. Some of the Vergueiro rifles were sold here in the 1970s, do not command high prices even though they are magnificently-made.
Rumania was on our side, used their official 1893 Mannlicher bolt rifles in 6.5x53R..... and anything else they had been able to capture during the First and Second Balkan Wars.
Serbia, which actually started the war, ended up somehow on OUR side, used Mausers from 1878 onward, older ones in 10.15mm, the later ones being in 7x57.
Russia had its Moisin-Nagant Model of 1891 but also used huge numbers of ancient 10.4mm Berdan II and even Berdan I rifles, due to scarcity of the MN. They also bought 2/3 of the total of all 1895 Winchesters, as cavalry rifles; these were at least in 7.62x54R. The MN went into production at Remington and New England Westinghouse and Russia took anything they were given that might go bang, By 1942, Russia had 2/3 of all Canadian Ross Mark III rifles built! (Hmmmmmm....... maybe THAT's why they hate us!)
With factories on 3 continents, the British Empire was able to standardise on their latest rifle, the Short, Magazine Lee-Enfield Mark III, later upgraded (some would say downgraded) to Mark III*. They also used everything going right back to Martinis for one purpose or another, hd close to 2 million P-'14s made in the USA and the Royal Navy and RMLI ended up using Canadian Rosses until 1945.
Canada and Newfoundland (then an independent country) both used the Ross Mark III rifle in .303", but both were converted in the combat zones to the British SMLE. Those of us who respect the Ross are still p*ssd about this but, then, it has only been 98 years.
Germany's main rifle was the Gew. 98 but they also made very large numbers of the Kar98aZ, which became the prototype for the 98k they used in Hitler's war. They also used vast numbers of their 1888 model rifles, at least until they were replaced by new production. Most of the 1888s went to Turkey, where they served until the 1970s. My old friend Jack Snow was in a POW camp in Ostpruessen for a good chunk of the war, being guarded by an old man with a rifle "with a bore the size of a sewer-pipe", so we know that some of the Mauser 1871/84 rifles served in rear-echelon units. The Gew 1871 and Kar 71 were standard rifles in the Kaiserlich Schutztruppe in East Africa; they only laid down their weapons 2 weeks after the war ended in Europe.
France's official rifle was the Lebel, but it was slow and difficult and expensive to make. Far faster to build was the Berthier, so far more Berthiers were built. France also relined a lot of 1874 Gras rifles from 11mm down to 8mm and you can still find these. They bought a huge number of rifles from the USA, including a very large contract of Remington Rolling Blocks in 8x50R Lebel, along with Berthiers built in the US. A French rifle VERY little known (and very few made) was the A6: a semi-auto firing an advanced cartridge. They also made the St.-Etienne 1917 and 1918 semi-auto rifles.
That pretty much takes care of the MAJOR players..... although it does lengthen your list somewhat.
And don't forget the 40 or so MINOR nations which took part in the "War to end all Wars". They include neutrals such as Norway, keeping the pledge made to King Harold in 1066 and not fighting against England.... and even aiding Britain navally; Venezuela, neutral but supplying the Allies with oil, Azerbaijan on Germany's side along with the Dervish State and Jabbal Shabbah: modern Saudi Arabia, Syria and part of Jordan; Allies such as the Republic of China, which did not have the resources to do much at all, Nicaragua; Siam; the little Europrea arms powerhouse of Belgium...... mostly occupied by Germany and one of the main battlefields of the War. And so many others. It was a WORLD War. The first ever. And it haunts us even today.
Good luck.
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