Can anyone help me with adding attachments to a gun?

Do you have the gun, and what kind of attachments are you trying to make?

For a barrel, increase the power. For a silencer/suppresor, change/remove the firing noise. For a bayonet, change it's melee damage. I don't know about scopes or grips, though. Also, this assumes it's a normal ranged weapon

Also, these would be diffrent elements

This may be tedious, but you could just add all the combinations you want as their own item with the properties you want, and use crafting to switch between it. Perhaps make a new item just for separation of specific parts

That's possible, although you could try and make the attachments like enchantments, except by making them removable. I would also say not to make the guns with ranged items, because those only give you bow physics.

If you increase the power, you get physics more like guns. The higher the power the faster it goes, whilst the fall speed stays the same

Also, turn on keep firing for a faster fire rate

Even if you do that, it still does the animation for a split second. The arrow power also increases both the Speed and Damage of the projectile, which should be separate options. It can't do things like rate of fire or bullet spread if you wanted to make a shotgun. The new velocity and rotation procedure blocks should help with that.

Try having the base model having all the attachments and, for every unique combination, making their bit of the texture clear