Modding Help Is it possible to create custom projectiles in a Mod?

So, I've been making a mod that required custom projectiles. I couldn't get the projectiles to work though.
I did find certain posts claiming that it was impossible, and others that said it was, but not explaining how.
Can I get a definitive answer please?

 

Some of them require animations, projectiles require frames files, and some require effectsource files. I encountered the same issue making liquid guns.