Walther PP 4mm conversion

We get to kill two birds with one stone this time around: We'll take a look at the very popular Walther PP pocket pistol as well as it's often hard to locate 4mm conversion unit. Indoor shooting was once a very popular pastime in Europe and it was not at all unusual to fire so called Parlor pistols while visiting one's host...after a few beers and chatting about politics of course (anyone think that Adolph Hitler charactor might actually become chancellor? NAH!). Although dedicated firearms were made for this purpose many well heeled folks would purchase 4mm adaptors which allowed them to enjoy their pistols indoors while using these low powered cartridges and still retain the ability to fire standard ammunition from their pistols when needed. Perhaps the most well known of these low powered rounds was the RWS M20 centerfire which fired a .157 caliber round lead ball weighing 7 grains. Both Mauser and Walther produced adaptors to allow this tiny cartridge to be used in their pistols and we will look at the kit intended for the Walther PP.

The basic Walther kit consisted of a 4mm rifled barrel insert with a brass screw to secure it into the .32 ACP barrel, a small punch, cleaning brush and rod and a series of cartridge adaptors into which the 4mm M20 rounds were inserted. Once a sufficent number of adaptors were loaded they were inserted into a standard PP magazine and the weapon loaded in a normal manner. Since the 4mm cartridge used a small pistol primer as it's source of power this energy was not sufficent to cycle the PP's action automatically so each new round would have to be cycled into the chamber by hand retracting the slide to eject the spent shell. This process would then be repeated until the supply of cartridges in the magazine was depleted. After firing, the hand ejected cases were gathered up and a punch was used to remove the spent 4mm M20 from the adaptors. Although this particular kit was the most popular there was also a second type of 4mm conversion which did away with the adaptors all together. In these units the 4mm barrel insert had a short loading shelf at its breech and a single 4mm M20 round was placed onto this shelf with a spoon type device. After the pistol was fired a long rod was inserted into the muzzel of the pistol and the spent M20 case was driven out of the barrel after which time another 4mm round could be inserted into the barrel using the supplied spoon. Due to their slow rate of fire this second type of kit was not as popular as the first.

These 4mm conversions survived into at least the 1960s although sales numbers remained small. An interesting pistol, the ERP74, was developed in the 1960s by the German firm ERMA which duplicated the Walther PPk except it was chambered specifically for the 4mm RWS M20 cartridge. While the M20 cartridge remained too weak to cycle the pistols action, ERMA developed a cleaver method to overcome this. The 4mm cartridges were inserted into their standard adaptors and then placed into the ERP74's magazine after which the slide was cycled to chamber a round and the trigger pressed to fire the pistol. After firing, the slide remained closed and the trigger is pressed a second time which caused the slide to retract and eject the spent cartridge adaptor. As the slide closed again it chamberd a fresh cartridge, the trigger was pressed again and the firing cycle continued until the magazine was empty. These ERMA 4mm pistols are seldom seen in this country.