So before you pick a locked weapon do you want to test it out? Well here is what you can do. Locate the file called unlocks.py it should be found down the path ....\Battlefield 2\python\bf2\stats . Find the line
defaultUnlocks = [0, 0, 0, 0, 0, 0, 0]
change it to
defaultUnlocks = [1, 1, 1, 1, 1, 1, 1]
Now in SINGLE player mode you can try out each of the weapons. BTW if you patched to the latest version EA fixed it so it does not work anymore. If you still want to test out and not downgrade your retail BF2 install simply D/L the demo and make the change to it. That's what I did.
Again this only works for SINGLE player games.