This last sentence provides more insight into a sound suppressor for the MP40 that can be considered one of the prototypes featuring rubber baffles. This design was tested but ultimately proved ineffective.

The images to the right show this model with rubber shims, which the Germans referred to as the "Russian System" because it was based on the design of the Russian Bramit silencer for the Mosin-Nagant M91/30. The German system also followed a similar principle: it first featured a larger gas chamber to allow gases to expand, followed by two rubber shims with a second small gas chamber in between. However, the silencer shown here is incomplete, as it is displayed with only one rubber shim.

A major issue with rubber shim silencers is that, due to the rifling of the barrel, which imparts a right-hand twist to the bullet, some of this rotational force is transferred to the rubber shim as the bullet passes through it. This could cause the silencer to loosen over time. To counteract this, the Germans used left-hand threading, allowing the silencer to remain securely attached while still being easy to disassemble for maintenance. Since rubber shims are consumables and need periodic replacement, accessibility was crucial.

For the Karabiner 98k (K98k) and MP44, the Germans used the Gewehrgranatgerät (GGG) coupling, which locked around the front sight base. The last image provides a comparison with a K98k rubber shim silencer featuring this coupling. However, with the MP40, this coupling could not be used due to the barrel rest. Instead, they opted to use the original muzzle threading, supplemented by an additional clamp at the rear, which fit around the front sight and could be further tightened.

The prototype silencer shown here lacks any markings, which is not uncommon for German silencers. Some K98k silencers with metal baffles are also devoid of markings. Regarding performance, rubber shim silencers were generally more effective in reducing noise than those with metal baffles. However, silencers with metal baffles had less impact on accuracy, as the bullet did not have to pass through rubber.

The prototype shown to the right is sometimes referred to as the Arado L41, as mentioned by Ian McCollum in his Forgotten Weapons video on the subject. However, in my opinion, the last sentence in the official German letter contradicts this claim.

To conceal the project, the sound suppressors were designated "Haube" (German for "hood"). Aside from the prototypes, a total of four different sound suppressors were developed for the MP40:

• L 41
• L 42
• L 43
• L 41 (new version)
  

The newer version of the L 41 was manufactured by Arado, and it is likely that the earlier L 41 was also produced by them. The L 41 had a cylindrical shape and was screwed onto the muzzle. It measured 43 mm in diameter, 295 mm in length, and weighed approximately 0.7 kg.

The L 42 and L 43 were developed by Schneider. These suppressors were also cylindrical, with a diameter of 45 mm, a length of 350 mm, and a weight of about 0.6 kg.

Firing tests at Kummersdorf

On December 17, 1944, and December 31, 1944, firing tests were conducted at Kummersdorf by the Ballistic and Ammunition Department of the Heereswaffenamt.

The first test, on December 17, 1944, aimed to determine whether firing an MP40 with a sound suppressor would yield the same results as firing it without one. The findings showed that the L 43, which had an 11 mm opening, performed without any noticeable difference. However, the L 41 resulted in misfires, as its opening was too narrow.

The second test, on December 31, 1944, led to a more concerning conclusion. The L 41 was modified to have an 11 mm opening—this was the new version. Now, its performance was comparable to an unsuppressed MP40. However, the issue lay in the connection to the muzzle. The MP40's muzzle thread was originally designed for a blank-firing adapter. This adapter was a small, lightweight device, whereas a larger, heavier suppressor required a longer thread engagement to ensure secure and stable mounting.

Without a sufficient thread contact surface, the suppressor would risk misalignment. If misaligned, the bullet could strike the baffles, potentially destroying the suppressor and endangering the shooter or nearby soldiers due to fragments from the suppressor being blown apart.

Additionally, both suppressor designs required special subsonic ammunition, known as "Nähpatrone". This ammunition featured a heavier bullet (9 grams instead of the standard 8 grams) and a reduced muzzle velocity (below 320 m/s) to prevent a supersonic crack—which occurs when a bullet exceeds 1,080 feet per second.

It is unlikely that these suppressors could fully eliminate the sound of gunfire. A more reasonable assumption is that they were designed to muffle the sound enough to confuse the enemy, making it harder to pinpoint the shooter’s location. However, it's worth noting that by this time, the British SAS and the U.S. OSS had already developed effective suppressor designs, some of which had fallen into German hands after being captured from partisans or commandos. (For example, the suppressed Sten gun.)