Originally Posted by
VAMarine
It's not just the slide you have to get the stop through, there's a oblong whole in the barrel link that the slide stop shaft needs to be inserted through.
In the picture posted above, the barrel's been pushed back too far and the slide stop shaft is forward of the link which results with the guide rod contacting the slide stop shaft instead of the barrel link.
When you're assembling the gun, make sure that the oblong hole of the barrel link is centered with the hole in the frame prior to inserting the slide stop.