@Meanlucario
 
Doing a quick search, it looks like forcefields have stopped material objects any number of times. Changelings, for instance. See also >>1536825 for a forcefield holding back soap bubbles, and Twilight puts up a shield to protect herself from incoming books in >>1477493.
 
So it looks like they’ve already done it, so projectile weapons would already be of limited use in Equestria. A pistol can deliver about 1000J of muzzle energy, while a heavy mace can deliver about 200J… but of course, the mace has infinite ammo. Hence why the changelings bodyslammed their way through Shining Armor’s shield instead of pewpewing it down, no doubt.

Preety good

@Background Pony #335C  
Have you tried using magic to stop bullets? Lasers are easy, but lead is another beast entirely.

I mean they already have magic forcefields that can stop pew pew lasers. If somepony invented guns I figure they’d get approximately one use before the forcefield spell was altered to stop lead.

Yet another day saved by Sparkles! The Wonder Horse!

Would have made the movie better if this was what happened.

Tempest: “I have a army with me and magic that turns anyone it touches into crystal what do you think you have that can possibly stop me?”  
Twilight pulls out a handgun and cocks it with her magic.  
Tempest: “WAIT YOU HAVE A GUN?! FORGET THIS GUNS ARE DANGEROUS! Come on Grubber let’s get out of here.”

And another day was saved by Sparkles the Wonder Horse!!!