I thought way too much into this, but here it goes.
So, I pondered about this for some time now, and I personally came to the conclusion that there would be a reasonably large variety of weapons, both old and advanced (For the early - mid 1900s) with a decent amount of them being capable of being manufactured even in a post-apocalyptic world. I'm talking about everything between wooden stock, clip-fed bolt-actions and revolver/self-loading semi-auto pistols to stamped steel assault rifles and full-auto machine pistols, and back again.
Rather than going completely into the madness that was my mind when thinking about this, I'll just go into the "jist" of it. Also, correct any lore bit that I get wrong, please -
The first thing I naturally considered was where firearms and ballistics would be after 50 years of continuous warfare (and keeping it as simple as possible), while keeping in mind that the war was devastating enough to end civilization. To do this, I started from the beginning, or atleast where Guns of Icarus' timeline begins to split. It's 1917, WW1 is somehow still in completely full swing, with myself assuming that the Russian Empire is still part of the Triple Entente and the United States still enters the war on the Entente's side. Weapons at this point are at the same point they were in history, with the primary rifle still being bolt-actions, self-loading pistols just starting to slowly replace revolvers, and the light machine gun still not exactly being light.
However, since fighting continues into 1918 and 1919, weapons like the American BAR, German MP-18, French RSC M1917, and the Russian Federov Avtomat see more action than they historically did, although the Federov a bit less so. The BAR would probably make an even more significant impact than it already historically did, possibly to the point that it kills the MP-18 and the SMG concept. However, that would be highly debatable, so I personally compromised that while the full barreled SMG dies, the machine pistol (Like the Mauser C-96) gains renewed interest, giving the self-loading pistol competition. The RSC M1917 seeing more action would make such a ridiculous impact, that rifles like the M1922 Bang and the such would probably be actually adopted.
The big thing however would be the Federov. While I personally doubt it'd be an immediate hit due to the impact of the BAR and RSC M1917, it'd be the bee's knees in the early-mid '20s, and everyone would be jumping on the assault rifle wagon. Although it'd probably take until the late '20s and potentially early '30s for nations to choose an intermediate round, build an assault rifle, switch production lines, then field it. The resulting rifles would probably look pretty cool though, T20 Garand made in 1930 anyone?
The 1930s would be where things get interesting though. The world has been in a continuous war for over ten years, and some of the smaller nations have probably fallen apart or been annexed by now. Keeping in mind that other things will have naturally advanced as well (Like tanks, planes, etc.), countries would probably be looking towards making more simpler and easier to manufacture firearms to ease up on the lumber and cost, meaning that guns at this point would possibly start being made with fewer internal parts, and be mostly stamped/turned/whatever steel later in the '30s.
Things would be rough by the '40s. A decent amount of Europe and East Asia would probably be abandoned due to war and random clouds of chemical death floating around. Nations would start designing/redesigning weapons that could be made in basements if need be and probably be back to clip-feeding in the early '40s, and by the late '40s they'd be back down to using high-capacity bolt-actions and revolvers/semi-auto pistols.
By the '50s, well, everyone would be too busy killing each other for food, water, or fun to design guns.
Now, what does this all mean for Guns of Icarus' time period? Like everyone else said, bolt-actions, lever actions, revolvers, and breech loaders would be the absolute norm. However, I could see late-war designed, clip-fed semi-auto rifles and pistols made entirely out of steel be somewhat common, full-auto machine pistols not being common, but not being rare either (They'd probably gain alot of popularity with aircrews), and full-auto steel rifles being borderline rare. I could also see guns with detachable magazines (Along with having extra magazines) or wooden stocks a sign of wealth (and a fashion statement).
All in all in a true jist, while I can see guns in Guns of Icarus being mostly old style stuff, I also think that more "modern" weapons wouldn't be less fitting.
