The PSDR 3 is a fully suppressed .45 ACP Smith & Wesson Model 625 revolver developed in 1993 by Joe Peters, under contract from Northrhine-Westphalia, for Germany’s SEK (SWAT) teams.

This photo is from Visier Special 6, 1997 (special edition of one of the three big German gun journals):

Click to expand.

Revolvers cannot be effectively suppressed because gas escapes from between the cylinder and the barrel creating noise (there are some exceptions). The big shell that encloses the cylinder of the PSDR 3 prevents the gas from escaping. This combined with a big suppressor and a subsonic .45 ACP round causes a noise reduction from 136 dB to 90 dB which is comparable to a CO2 air gun. Decibels are a logarithmic unit of measurement, so a 46 dB reduction is significant. In theory this gun should make a lot less noise than a suppressed semi-automatic pistol because there no slide slamming open and closed.

I had no idea such a gun existed until Sven emailed me. I really like the idea – German engineering at its best!
Many thanks to Sven, who blog at Defense and Freedom, for the scanned page and translation.