Challenger F. Sight Base Q??

[YEMX]

Quick questions for those that have and/or tinkered with a Challenger PCP rifle. 

    Can a barrel slide completely through the front sight base, or is there a stop?  Does the flat area near the muzzle act as a stop for the front sight base?
 
    I notice from the diagram there are some notches in the front of the barrel- are these for the front sight base screws?

    How tight does the front sight base fit on the barrel?

Thanks all!!