Prob a stupid question...but when I look at the schematics...cant quite get it straight.

Can someone explain to me how the bolt catch assembly keeps the bolt open on the last round?

Thanks

The magazine follower pushes up on the catch in the lower when the magazine is empty. When the catch is held up (by the followetr in the empty magazine) it mechanicaly blocks the bolt from moving forward. when the magazine is removed the spring pressure on the bolt from the buffer spring exerts enough force on the bolt to hold the catch up and prevent it from allowing the bolt to move forward.  The catch has to be manualy pushed back down by the release lever on the lower, or by pulling the bolt back off the catch to allow it to be pushed back down by the spring in the lower.