Do they 'plunk test' OK on your barrel? I experienced something similar when starting out with the 147grn truncated cone projectiles. The curvature of the TC's was less than previously loaded RNs (e.g. the TCs engaged the rifling at OALs at which the RNs wouldn't) and I had to reduce the OAL until they reliably 'plunked' and dropped free of the barrel. 1.135" OAL for Bayou 147grn TCs seems to work well in my Glock 19. I would load up some dummies at various lengths and determine what your Walther likes. As others have said the coated bullets need a little more belling or you'll scrape the coating.
