Well, that's an interesting reason for the request. Okay.

To start, we're almost certainly dealing with the rifles employed by the major belligerents. That'd be the G98 for Germany ("8mm Mauser," actually 7.92x57mm), the British Lee-Enfield (7.7x56mmR), the Mosin-Nagant for Russia (7.62x54mmR -- note also that this is definitely the most produced family of bolt action rifles in history), the French Lebel (8x50mmR), and the Austro-Hungarian M.95 (8x50mmR during the war, different calibers after). Note that the Austro-Hungarians and French used different calibers despite the fact that they had roughly the same dimensions. Mauser sold a variety of other rifle types to different states (Spain, the Ottoman Empire, Brazil, and Sweden-Norway come to mind). The Ottoman Empire, though, essentially bought whatever the Germans had lying around throughout the war, so they used a pretty diverse array of calibers.

Beyond the rifles, the 9x19mm Parabellum, the quintessential "9mm" round, achieved widespread popularity in Germany during WWI. I'm not sure how eager you'd be to make these yourself, though; you can go to pretty much any hunting or gun shop and buy a box of them at a pretty reasonable price. The 7.63x255mm was another popular German pistol round during the war; it's used by the C96, the gun that served as the basis for Han Solo's blaster (and which, on a slightly more serious note, had that wooden stock so it could function kind of like a carbine). Similarly, the FN Browning M1900 shows up here, so the .32 ACP needs to be mentioned (the French Ruby was essentially a clone of an M1900 variant, if such a term can be applied to a gun produced by a number of Basque pistol manufacturers who didn't standardize between themselves). And, in the interests of gun nuts cursing my name, I'm obliged to mention the M1911 (.45 ACP), which, as the name suggests, is also contemporary to the war.

And while not "popular," if popular is defined by sheer quantity used in the war, the 13.2mm TuF has to be mentioned. It's from the T-Gewehr, the very first anti-tank gun. That bullet is actually bigger than a .50, if you don't know the conversions. (.50 = 12.7mm)

I would be remiss if I failed to mention machine guns. The German MG 08 used the same 8mm Mauser as the G98, the British used a number of different machine guns (Hotchkiss rechambered, Vickers, Lewis) in .303 British, the oft-maligned French Chauchat used the 8mm Lebel just like the Hotchkiss did in French service, and the American Browning used the .30-06 (like their Springfield rifles, which I didn't mention because they show up so late in the war).

I'm also obliged to add a disclaimer: because of the staggering size of the war, just about every functional rifle produced after the advent of smokeless powder was used. Even the G71, which was by that point absolutely ancient, saw use in Serbia in WWI. To put that into perspective, the G71 was a black powder rifle. That's how old the thing was. The Serbs did refit it to fire the 7mm Spanish Mauser, but that retrofit didn't cover the entire inventory and, given the situation in Serbia, it's rather likely that the black powder versions still saw action.

I'm probably missing some. WWI saw the use of so many different weapons (and therefore calibers) that it's nigh impossible to remember all of them. But I think this has provided a decent summary of the most widespread calibers.

Oh, and I'd love to see these things when you finish making them.