How to achieve the Arms Dealer Trophy in MW3

Yes. Trophies can be unlocked by a group or in solo.

It's normally best to buy all weapons by a group to unlock this trophy, to achieve this trophy effectively, follow this method:

• Be fair when opening doors, each player should take turns to spend their points when opening doors.
• Be prepared to gain many points before going out to buy all weapons.
• When ready to buy all of the weapons, use a grenade on a group of zombies (when near to end of a round) and make them legless which will leave you precious time to save running around to buy all weapons (including you and your friends/team mates to spend points towards the doors).

You may need about at least 10,000 roughly to spend on doors, and at least 12,000 to 15,000 (from what I remember ages ago). Which comes a total of between 22,000 to 25,000 points.

The Collector trophy does not require upgraded weapons or weapons from the mystery box.

This may be quite hard to achieve unless you have a good motivation of escaping and surviving from a large swarm of zombies and also taking them down, be sure to make good use of your grenades for high points when used against a swarm of zombies!

Optional: If you may want to test your luck, you can use the mystery box for a good, powerful weapon like the Alien/Ray gun could wipe out a lot of zombies in a few shots (better when upgraded) which will make you kick ass!

Tip: Don't camp. :)

I'm copying the below quote from this link

Under Options on the main menu there is a new area called Extras. Under Extras you will find Statistics which is what you need to view to get this trophy. To obtain all of the stats you must race on every track, yes all 24 tracks, and fire and absorb every weapon. You will notice that is shows your progress on Arcade Perfect on the first page, don't worry you don't have to earn Arcade Perfect to get Stats Life. UPDATE: You must do a split screen race on any track as well. If you don't have two controllers you can use either a Guitar Hero guitar or your PS3 DVD remote, just set the remote to controller 2.

If you follow a link, the author makes a special note about the split screen race being necessary. See if that helps.