1. What the hell is wrong
Try to convert your model to 3DS, or if it's in 3DS, then to .X. If both model formats do the same, then start flipping the normals until it's fixed.
2. Another way to position the gun while you move
Something like this:
Gun=1
position object Gun camera position x(), camera position y(), camera position z()
set object to camera orientation Gun
Move object Gun,5
3. Where i can get another MP5 model
http://www.3dcafe.com
http://www.turbosquid.com
Counter-Strike 3D Models...just find a conversor from MDL to 3DS, X or TrueSpace 3D object file format and use it. You can use MilkShape for it

.
4. Where i can get another free modeling program
Take a look at my Game Programming Kit For Beginners, there you'll see a HUGE list of free and cheap 3D modeling programs
http://darkbasic.thegamecreators.com/?m=forum_view&t=30080&b=10
I hope to have helped...
edit: grammal errors...
:: Pentium 300 Mhz, old 8Mb video card, 64Mb RAM, 5 gb & 1.6 gb HD's, W98 SE, Sound Blaster AWE 32 ::