Since I wrote a bit about these a while ago, I thought I’d better paste it into this thread as well (also because I had some trouble locating it twice already :) ):
Something to remember is that the Luger/P08 chamber design was a stepped one, intended to create a gas seal, similar to that present on it’s forebearer, the 7.65 para version.
This means that a sub-calibre insert, which follows the shape of the 9mm para cartridge can easily get stuck and would be difficult to remove. A solution, of course, is to shorten the subcalibre unit so that it’s top will not get stuck on the stepped part of the chamber.
The chamber step is located at exaclty 5 mm from the barrel end of the chamber. A 9mm para round, fired through a luger, will always show a rimmed deformation, 5 mm from the top of the cartridge, as a result of this design feature.
I’ve had a lot of fun using a laser sighter that worked by inserting it into the chamber. In the P08 it got stuck and I had to hammer it out from the barrel end with a cleaning rod.
I digitally restored a 1930s Erma subcalibre brochure a few years ago, They used a loading spoon construction for the M20 ammunition (which was popular as it could be used for indoor plinking) and a separate barrel/insert with chamber piece and replacement toggle for a full-auto .22lr version.
If we go back to 1920 we see that the first versions of the subcalibre inserts for semi-auto pistols were produced by G. Genschow & Co (GeCo) in Berlin and W. Eblen in Stuttgart. These were set up for 4mm rimfirerounds. The bore of the barrel insert was excentric of design, so that the gun’s original firing pin would touch the catridge rim. This design feature also meant that no inserts for 7,65 calibre pistols were made, as the insert’s barrel edges would be too thin.
In order to solve this problem, G. Genschow & Co and RWS constructed a new type of 4mm round which could be used as a centerfire or as a rimfire cartridge, effectively removing the need for the excentric barrel design.
RWS and GeCo both used, as Erma later on, a loading spoon for the 4mm catridges.
In the mid-1930s we also see the subcalibre inserts for hunting rifles getting more popular, like the Columbus barrel, made by Burgmuller & Sohne. It was clamped into the rifle barrel using a leaf spring fastening method. Sempert & Krieghoff also marketed a barrel insert for their hunting rifles, called the ‘Semer-Schonzeitlaufchen’ which used a separate fastening method. Both were meant for the .22lr round.
Another version was the Horrido-Einstecklauf made by Eoschke, again for the .22lr round.
Most of these sub-calibre inserts had one thing in common: They were loaded directly without the need of a separate retaining piece for the small calibre round.
An exception were the subcalibre set called ‘Champion Zimmerschiess- und Ubungsapparat’ and the ‘Rekord-Zimmerschiess-Apparat’ made by W. Glaser. This set, which was available for hunting rifles, carabines and pistols of calibres of 6,5 mm and higher, and made use of a separate retaining piece in which the subcalibre round was placed, this retaining piece was then chambered, like an ordinary round.
If we look at a 1936 RWS catalog, we see that they offer the subcalibre set with loading spoon for most pistols in 7.63, 7.65 and 9mm. Their subcalibre insert had been patented und D.R.P. 365 264 and DRGM 778716.
Pricing varied from to 6.30 RM for rifle inserts to 9.60 RM for pistol inserts.
An interesting table shows the pistols for which they can deliver inserts:
Bayard in 7.65
Bayard, model 1910 in 9mm Bergmann
Browning M1900, 1910, 1910-22 and 1920 in 7.65 and 9mm short.
Colt .32 in 7.65
Colt 1911 in .45 ACP
DWM in 7.65
Dreyse in 7.65
Frommer-Stop in 7.65 and 9mm short.
Langenhan in 7.65
Mann in 7.65
Mauser C/96 in 7.63 and 9mm Mauser Export
Mauser M30 in 7.63
Mauser M30 in 7.65
Ortgies in 7.65 and 9mm short
Parabellum in 7.65 para and 9mm para
Rheinmetall in 7.65
Sauer & Sohn M1913 and M1930 in 7.65
Savage .32 in 7.65
Star, several models, upon request.
Steyr M1911 9mm Steyr
Stock in 7.65
Walther, old model in 7.65
Walther PP and PPK in 7.65.
–Moderne Faustfeuerwaffen und ihr gebrauch, Gehard Bock, 1920.
–Waffenlexicon, Richard Mahrholdt, 1937.
–Erma subcalibre insert brochure, 1930s.
–RWS catalog, 1936.
–RWS catalog, 1922.