New Update 1.1!
WebGL Demo
WebGL Demo 2
UnityPlugin Demo

Particle Bullets is a particle based projectile system for 2d and 3d. A gun prefab can be attached to a GameObject and the gun fires from its forward. It uses Unity's legacy particle system to emit projectiles and solve collisions and is set up such that all the variables can be tweaked to get unique weapons.

Source code is free to use and modify, so this package is great for jumping right into level creation, boss design, and enemy design.

Dependency: Property Backing Field Drawer (free)