I am not experienced enough with all of the code for this mod to know what is possible and what isn't possible but I will leave a few suggestions that I believe will enhance the addon creation experience and allow for more diverse features without needing to go through the hassle of coding them in an addon.

• Fire Mode Selector (auto/semi - ability to designate default fire mode)
• Burst Fire Mode (Fires a customizable amount of rounds before experiencing a delay, then another volley of rounds)
• Pump Fire Mode (Fires a round then plays a "shotgun cock/pump" sound effect before being able to fire again.) -- this would allow for pump shotguns to be done correctly without needing to combine a pump and fire sound into one audio file
• Fix trails and/or allow them to use certain particles from Minecraft's particle catalogue
• Customizable muzzle flash
• Attachments have their own data in the data folder to make creation and editing outside a developer environment easier (guns and their data can be tested outside of a development environment using datapacks and texture packs)
• Allow for "reload type" - can be set to "magazine" and be more animation-friendly.
• damageReduceOverLife goes from boolean -> int and accepts a value in blocks. It would also be nice to designate when the bullet starts to reduce in damage and the percentage that it reduces as it falls off.
• Make spread not have visual horizontal recoil characteristics and make a "horizontal" and "vertical" recoil component. ("Vertical recoil" is just what "recoil:" is now.)
• Default ADS speed configurable in weapon data
• "suppressMuzzleFlash" attachment attribute
• Aiming decreases spread and recoil instead of just recoil
• Introduce "weapon weight" that decreases movement speed when weapon is equipped
• Allow for most any weapon data characteristics to be applied to an attachment as an attribute: e.g., "test attachment 1: increases ADS speed, decreases recoil, decreases spread, increased range, increased reload speed"

It also would be nice if the fire rate could be more than just an integer to allow for more diverse fire rates, but I understand that the fire rate is tick-based and "fixing" this would require rewriting a lot of code.