I don't know if anyone knows or not, but are there any stats on when it is best to switch weapons for challenges and such to speed up leveling?  I can't decide if it's better switching after unlocking the easy challenges/attachments, or if it's better to switch after mastering a weapon.  Mastering takes more time/kills, but gives a ton more experience, so I was just wondering if anyone out there had any data on this.
 
Even if you don't have any real data to back it up, what has worked best for you: switching often or mastering weapons?

Mastering Weapons for sure. I found you can prestige faster if you master one or two specific weapons. I'm going onto my 5th prestige and can confirm that it has taken me less time to prestige each time as i have concenrated on 1 or 2 weapons.
 
Give it a go and see what works best for you.

It sucks for me because the weapons I do want to level up the most are the M16 and ACR :P

I would love if there was a playlist or a series of playlists that gives everyone all weapons and upgrades and no ranks. I am level 45 in mw:2 I don't play much, but would like to skip all the leveling shit

Just prestiged, so I'm gonna try mastering my weapons this time through then and see how that goes.
 
I guess the good thing about mastering is I don't have to worry about using crappy guns as I level.

I constantly use a specific weapon. What i recommend is that you master close range and mid range weapons. That how it works for me. It does depend on the player and what they want to do, just be smart in choosing your weapons to your style of play.