Yeah, Walther LGV & century.. the later is a full 30mm, the former the same gun sleeved down to 25.
Add a longer piston rod for 6Fp with the sleeve and you have 6 Fp covered. Job done.
More like 20 FP max though, but that's just a stroke issue. Build the gun with a longer cylinder, and longer stroke in the 30mm version becomes easy.