Could i send in a 3d printed gun prototype to a firearms company to see if they like my idea?

Nope. A reputable company would send your unsolicited prototype back unopened.

You should spend some time reading and looking at web sites that deal with guns, gun manufacturing, and prototypes. Because most of the big companies - have enough new 'ideas' to last them the next 20 years. Go visit the Ruger web site and read what their CEO has to say.

Go do some research on JD Jones of SSK Industries. He started out making custom Contender barrels in all sorts of wildcat and weird calibers and cartridges - and turned it into a full grown company. If your idea is good - you should make a working prototype - and then walk around and talk to whom ever will give you the time of day at the Las Vegas Shot Show or similar other events. You need to do the 'grip and grin' - learn to memorized folks names - and learn how to name drop so you can eventually get in with someone that will take your idea to the next step.

You dont want to just mail your idea off willy nilly.

You can, but most if not all companies will send you "design/idea" back to you, without looking at it.

Why?

Because they do not want to be sued by you years later if they come up with the a similar or exact same idea on their own and start selling it. So unless they ask you to send in your ideas and designs, you will not be getting your "products" even commented on.

wow... first kid on the block with a $15k 3D printer and the stuff to make guns already.....

on the odd chance you got in front of somebody to pitch the idea... they would be more interested in "proof of concept" drawings and what makes your idea materially better or marketable.

and any company with ethics would send back any un solicted ideas for legal reasons

Yes, but fire arms are more than just "does it look cool?"

Does it fit well? If we use X material in this design, will it be durable? will it be light or heavy? does it feel well in the hand? is it concealable? is it cheap/cost effective to produce? Does it match our style?

Look Austin....do whatever you feel like...or do you need OUR permission, first ? If you don't have a copyright / patent though, they may screw you out of millions....millions I say !!!