The best way to improve or learn to model weapons is to recreate an existing gun, something you can get a lot of media for, then try to recreate it in as few polygons as possible.
Even if you never plan to use real world guns in a game, you should do a few of those, because loading up a modeller with only a rough idea is not gonna give you good results until you can see the model from a realistic standpoint. I mean a lot of people make sci-fi style weapons without considering what it all does, the weapons in use today tend to be as compact and as light as possible, and this is something that you should learn to replicate by making real weapons.
Take the human weapons in Halo as an example, the shotgun, handgun, and rifles are all fairly recent looking, they work just like traditional weapons, the more rediculous ideas are reserved for the non-human enemies. If you want to make a futuristic shotgun, then take the modern day combat shotgun (like in BF2), and consider what would evolve - say the ammo delivery system, change that to a magazine, and the shell eject, maybe have the shell eject neatly, pushing a little metal flap as it goes out. It the details like that which are easy to miss when designing your own guns that makes the difference. By studying real weapons and recreating them, as functionally as you can, you end up being able to come up with believable designs that look good.
Maybe the easiest weapons to make are grenades, but theres a lot you can do with a simple frag to make it a great weapon. For instance, if you properly model the pin and pull mechanism, you can have it work just like a real grenade, have the ring-pull bob up and down when a character is running - things like that make a lot of difference.
I know it's only your 4th gun model, but your work is gonna be limited until you start designing the guns as if they already exist, even if they don't. Small realistic details and moving parts is the only way to make your guns believable, and there's tons of guns out there that can give you ideas. You might even have an airsoft pistol or something that would be great for copying, they tend to have similar moving parts to their real-life counterparts, even the really cheap ones.
We're going down... in a spiral to the ground...
