>>529734803
Just in case you didn't know because you're a warframe refugee or whatever, you get XP for your guns equipped when you kill things with abilities, too.
And if you are from really far afield, yes, you do get all the XP from enemies your allies kill so long as they are within 150m of you.

If you can find a pub supply defence with friends who are set up good, those give some of the highest weapon XP per minute you can get in this game.