KNEX Bullet Lock - Design Help?

I am trying to design a bullet lock for my new gun, but I am having trouble
The original 3 prototypes miserably failed and I need help. I don't want to lose range or power by adding one...

Here Are some Pics. Note the fact that it is Dual-Barreled.
http://dl.dropbox.com/u/18474025/gun_1.png
http://dl.dropbox.com/u/18474025/gun_2.png

Picture of KNEX Bullet Lock - Design Help?