I’ve been thinking about adding some shotguns to my collection but I know nothing about them. Any recommendations on shotguns used during WW1 or WW2 that would be a nice addition? Anything to avoid or look out for? Thanks.

There's a good article in this month's American Rifleman.  Check it out.

Look out for fakes.  It is not hard to build a "trench gun" that's not original.

Do a lot of research before you buy.

Some experts believe there are more Fakes than Real 1897 Trench Guns.

Bigger_Hammer

If you haven't looked at pricing(realized prices from sales) pepper your angus...that said 1897's model12's,savage autos& Ithaca 37's were used in the time frame you're looking at. Maybe a few more.

I'd buy Bruce Canfield's book on U.S. combat shotguns and memorize it as well as taking a deep dive into some of the online groups before spending money as fakes abound. Luckily most models have some obvious tells on whether it's a legit Trench or Riot gun or a reproduction built on a commercial gun. I settled on a Stevens 520-30 Trench gun, I intend to ad a 1897 at some point but I've never encountered one that didn't have issues.
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I have mostly fakes or replicas that scratch the itch.

Many of the guns (Remington model 11s, 31, Savage 320/520/620s, and even model 12/97 can be had reasonably so as used hunting guns that are retired.  They are fun when you can cut them down and have some fun with them and not worry about historical cost from shooting and enjoying.

I find fixer uppers sometimes really cheap - often just a simple part to fix.

You can find vintage shotguns from 1930s, 40s at reasonable prices.  Found a 1942 made Winchester Model 12 at a show for $125, just needed a few small parts to fix.