Whilst I have never come across any pre-1950s German-made Blanks, the “BFA” (Blank Fire Adaptor) shown in the above quoted books does lead to an idea that the Blanks themselves must have had a very small charge of Powder. The Inner tube serves to reduce the gas volume of the barrel, possibly allowing only a very small vent, so that there was initially a very small amount of gas produced by the Blank’s charge. If the barrel was only restricted at the muzzle (as is normal in today’;s BFA (Movie) Guns and Modern Military BFA), the gas pressure would dissipate in the relatively large volume of the (MP) barrel, and there would not be enough back-pressure to drive the heavy bolt of a MP backwards.
Nowadays, all Military and Commercial (read Movie) Blanks in 9mm have a substantial charge of Fast Burning Powder, sometimes more than that needed to drive the normal ball projectile, and sealed behind a plastic Cup bullet shape (RWS & GECO & BakelittenFabrik) or by a multi-petal brass neck crimp (“Star crimp”) which is also lacquered shut to prevent moisture intrusion.
The parent case of a full profile blank is usually bottlenecked to form the “19mm Mouth” by a shoulder, and the rest of the Bullet profile consists of the crimp itself, or by a short section of “neck” with the crimp on the End.
It is a strange co-incidence that 7,62x25 Tokarev Blanks (steel case) will cycle perfectly in 9x19 Chambers, the shoulder of the Tokarev case being in about the same position as the Parabellum Mouth, and any minor tolerance differences are “ironed out” by the heavy bolt of the MP38/40 series of guns.
The “inner tube” volume reduction system is used by us in Movie Blank adaptation of .22 Rimfire guns (semi auto rifles and “converted to full auto” .22 guns.
This is because of the nature of .22 Blanks available commercially are either very underpowered, or the use of Tool blanks in .22 Rimfire gives the wrong sort of pressure curves. Thus to get sufficient back pressure and some flash at the muzzle, a .22 rimfire must have almost no “expansion” volume between chamber and muzzle; only a very small exit vent.
Of all the guns used in Movies, the .22 rimfire is the most difficult to convert to blank fire with reliable self-loading operation (Pistols are slightly easier, having shorter barrels).
Manufacture of .22 Rimfire blanks by a commercial factory is a very difficult operation, given the rimfire nature of the shellcase, and the forces used to crimp the case (starcrimp). It is the only cartridge in Movie use that is not loaded by the Movie supply houses, unless they have access to a commercial factory of Tool cartridges or similar Rimfire blanks.
I have seen another photo of a pre-war screw-on BFA for MP38/40 guns, it looked like a “funnel” with a locknut, and screwed onto the M16x1 Thread of the Muzzle. Whether this was a “Bullet shredder” for wooden or paper mache false Bullets, I don’t know. ( It seemed very similar to the Locking device used in 4mm Inserts for P’08 and P’08 lange conversion kits)
Going sideways to another “MP”, the MP43-44-StG44, there was a Wood bullet blank for these guns, and obviously a screw on BFA here as well;
But these blanks functioned on the same principles as the 7,9 Platzpatrone used in MG34/42 etc, the wood giving enough backpressure to allow “Port pressure” for functioning to ensue, and then be shredded by the reduced vented cone of the BFA.
One of the most notable Reloading die makers in the USA made a strange mistake in designing their dies for Blank case making for the 7,9x33 cartridge…they copied the tapered neck Grenade-launching Blank of the PP43 cartridge, and not the classical star-crimp equivalent used in all-metal blanks…with resultant difficulties in reliable manufacture and also functioning. In our manufacture of PP43 Blanks, we use the said dies up to the second-last crimp operation, which is done by a normal star-crimper, followed by a finisher ( complete closure) and lacquering.
Anyway, just a Movie Blank supplier’s take on the various types of Blanks, especially those used in 9x19 Para—we use Fiocchi Military starcrimped blanks for almost all our SMGs, although for some (Owen Guns and MP18/28 & others with very heavy bolts) we make our own from either 9mm Win mag cases or cut down 5,56 Military (Blank) cases, as the insufficient “shoulder” of the Fiocchi blank sometimes gets “pushed over” and the blank goes too deeply into the chamber to be discharged (very common in some older Owen guns).
With the in-house made ex-5,56 blank case, a more substantial shoulder is formed, by first expanding the cylindrical case to a little over the true 9mm case mouth diameter, and then using custom made “blank forming dies” to give a good shoulder to the case, replicating a “bottlenecked” case, which will seat even in a worn chamber of an SMG.
The manufacture of modern Military and Film blanks is an area of “black arts and other arcane cartucherie” ( at least to us).
Regards, & Happy New Year