It used to be the case that buying the pistol clip extension upgrade (in Ristol) would do nothing. In 2018, the game got a patch (1.5), apparently to fix this bug- to quote the changelog, "Fixed crucial bug which messed up weapon purchase." The problem is that the clip extension upgrade still doesn't work. After buying it, the pistol still has a maximum capacity of six shots- though I think the sound it makes when fired is a different, deeper sound effect. It's hard to tell if it's actually doing a different amount of damage (since you don't see damage numbers), but the deeper sound implies some sort of upgrade to me. Maybe the patch accidentally made it so that buying the clip extension applied the wrong upgrade? Maybe the piercing upgrade? That's better than not applying any upgrade at all, but it's still a bug.