Forgive my gross reductionism here, but I'm going to try to put your problem into my own words and see how I'd approach it. I hope this is helpful.

Currently the question is "how many guns (and what names) do I need for fun progression." Put another way, you are trying to generate a list of weapons and map them to "stages" (for lack of a better word) in your game.

First thing that occurs to me is to flip the perspective on its head to see if that helps.

Reform the question thus: try to generate a list of "stages" (areas, regions, maps, dungeons, whatever the case may be :)) and then map them to weapons. Maybe the starting machine gun is a lot of fun for the first one or two areas, but then either you want a change and/or need a change because you want to add a new monster mechanic (e.g. zombies with riot shields). So, underslung grenade launcher OR armor piercing rounds OR whatever.

I have also ran into the same problem when i did my design.

And I can tell you I have atleast spent a week daily researching about real life guns trying to make sense of the scaling structure,

and my conclusion is, once you step your foot into using real guns like SCAR-L and NATO ammo, you wont get far before you realize realism has no space for scaling, i gave up.

of course it would look stupid to use real life guns name but shoots laser beam or what not. but you can keep the look (twist a bit maybe) and name something sci-fi, or cool name like Excalibur, ragnarok, etc. OR, how about scaling the ammo?

Realistically speaking there are only 4~5 varieties of ammo dominating 99% of the gun technology, and they're NATO only different in size. with some exception like FN series. And things are so for a reason: the design utilized the best physics can offer for that size and weight.

its hard to be creative in realisitic setting with guns or ammo. it is not a technology that can give and take by tweeking here and there, thus variation in stats is almost impossible. besides, who crafts the guns? obviously it has to be a mass production industrial product, distributed through a certain system. While sword and other traditional weapon, they are made by artisans, or legendary ones kept by the dragon. somehow dragon or zombie dropping a legendary gun doesn't make much sense.

Spiral knights is an MORPG that has a lot of guns involved, with a scaling in power, and crafting system. give it a look perhaps it might inspire you. Other RPG also involves gun like Aion, C9 online, Firefall, PlanetSide 2, Guild wars 2. but ofcourse, they are all non-realism approach.

you might also, for a class of firearms, have several ammunition with special traits : regular, hardened, explosive, armor-piercing etc... special ammunition can cost lot more money than regular, and thus requiring high level area, where there is lot of money to loot.