I understand it was a little confusing with the wording. I'd, however, be an idiot to suggest, that shotguns can't be upgraded. Being clearly evident in every workbench. They can't have mods/offhands, which offers so much utility.
I want to clarify, since you specifically said "you", when you replied to the message. I made the tips, as I didn't see them written anywhere and I thought them highly useful to new players. Investing in agility with your SP makes the world of a difference in most fights. I'm at the end of my run according to the walkthrough. I used the handgun all the way, with only a few exceptions. Using the most powerful weapons upon acquisition, will economically leave people in the rut. Possibly making people feel, they have to use an assist, to get back on track (there are other ways out, I know), baring them from the true end route. There wasn't anything about the bullet economy, so I thought I'd chime in with a few words.