Try getting another m4 mag to test with. Hi-cap mags can be troublesome sometime. Did you wind it until it started clicking? Like 20-30 times? 4-5 bbs falling out when you remove the mag from the magwell is normal for all spring loaded mags. If you don't want to lose these bbs and don't want to dry-fire, you have to turn the gun upside-down.
The forward assist is just for looks.