I know there are already a couple of gun mods, but bear with me. Besides, an existing gun mod could include this in their mod anyway.
The idea comes from the game Receiver, which is a very simple (yet complicated) shooter in which you manage every minute detail of your gun. When you reload in Receiver, you have to manually open up your gun, and move every single bullet into place, remove the safety, get it ready to fire, ect.
How would this fit into Minecraft you ask? My idea is that each gun would have its own GUI. Each attachment and bullet would have its own slot in which it must be placed. The GUI could be bound to a key (R would make the most sense) and you would have to press a series of buttons also in the GUI for actions such as removing the safety or cocking a shotgun, ect. Therefore, you would manage every minute detail of the gun just like Receiver. Since Minecraft is about realistically managing your inventory/resources, this mechanic would fit right into the game nicely. Each bullet would have its own slot, each mag would have its own slot which would open up slots for bullets to be loaded, each action would have a button to go along with it, and you would be able to micromanage everything.
You might say "This seems like a nuisance! Why would anybody want to have to struggle with this?" Well, I have an answer. Just like in Receiver, you would gain the benefits of managing every detail of the gun. For example, you could fill only certain slots of a revolver with bullets, and play Russian Roulette by pressing a button that would rotate the chamber randomly. You could give somebody a gun with no bullets to fool them. Reloading would become a game of skill, and you could accidentally lose bullets on the ground and have to pause the process.
The best part is that it's actually possible, and that a modder with only a good amount of experience could do it. I have no modding experience at all, but the whole mod itself would just be GUI work and of course putting the guns into the game as items along with all the gun materials (such as bullets or attachments) with other (more advanced) things down the road. The mod would be fun in many ways, especially if there was multiplayer support. Imagine playing Russian Roulette with friends, or having a realistic firefight utilizing the obvious destructive environments of Minecraft!
-Guns that each have their own unique GUI, ammo type, attachments and values (such as damage or block breaking power).
-Bullets can destroy blocks and are actual entities fired from the muzzle.
-3D models and custom animations (I doubt it).
-Realistic knockback and recoil effects.
-Realistic particle effects.
-Bigger guns may slow you down.
-LEAVE A COMMENT WITH IDEAS TO ADD TO THIS, I WILL GIVE CREDIT
I've had this idea in my head forever, even before Receiver. I doubt many people will see this thread, much less try to make this idea reality. But you have to admit it would be awesome. Leave feedback below!
T1A has most of these features WIP, though I haven't been working on guns recently.
Currently I have, full ricochet, muzzle flash, bullet drop, bullet deviation, recoil, HE and tracer rounds, shotgun, semi-auto, full auto.
Theres multi ammo types per magazine in the order you load them, but i dont have a loading system yet, the test code works though. Also theres full simulation of the loader, when you change mags the round in chamber stays there. When it jams you can sometimes rack it to unjam it, you can also rack it to pop out the round in chamber.
Eventually ill add attachments like silencers, and a crafting system where you can strip a gun to refit it.