The Walther PP series barrels are NOT just slid in and retained by the pin.
The barrel is actually pressed in with a special press, and it WILL NOT just slide out.
It requires a special press and support jigs to remove the barrel, and a different support jig to support things while the barrel is pressed back in.
Try to do it without the press and support jigs, and the frame can be distorted or actually broken or cracked through the barrel lug.