It would. This is why the people involved in the resistance needed to be creative. In the following entry I will focus solely on the case of Polish resistance as this is the one I am best familiar with.

In general, the production of the weapons on any considerable scale was limited to copies of British STEN machine pistol and a domestic 'Błyskawica' (Lightning) machine pistol designed for clandestine operations, as STEN, with its magazine loaded from the side was hard to conceal when loaded, while German were considered to be too heavy and bulky to be easily hidden (e.g. under a coat). Nevertheless, these weapons were initially constructed in few factories and workshops owned solely by Poles to minimize the chances of anyone connected with the occupational forces learning about this.

Now, the testing of the weapons was noisy, so the conspirators had to hide or mask it what was actually not that difficult as one might suppose. The trick was to make the existence of the weapons known to a minimal number of people other than those already involved in their production. The testing of the first prototypes of the new machine pistols was conducted in the woods surrounding military training facility in Zielonka, a small town near Warsaw. This, of course, was not considered a viable option for a large-scale production, as despite the seclusion and relative secrecy, someone could have became suspicious and the transportation of the assembled weapons from and back to the city was significantly rising the chance of being captured, even accidentally.

The first test shooting of the "Błyskawica" was conducted in the cellar of the All Saints Church at Grzybowski Square (the parson was already helping local Jews, so he was considered trustworthy by the resistance) as the deep cellar and massive walls of the XIX-century building could easily dampen the sound, especially given that the church was located next to the busy area. Additionally, the church was significantly damaged in 1939, so even if someone was able to hear the muffled shots, these could have been easily taken for the sound of repairs.

When possible, weapons were also tested in the well-isolated areas (preferably underground levels) of the various places they were assembled in. In most cases, the weapons and their components were assembled and stored in the hidden parts of the legally operating businesses. As they were usually various metalworking factories and workshops (most of the weapons were assembled in the Makowiecki's workshop producing metal fences and the factory 'Konrad, Jarnuszkiewicz & Co.' producing hospital equipment), the noises typical for metal stamping, grinding or milling were adequately masking the shots already muffled by the walls. Majority of testing was conducted in the second of the two locations mentioned above, and according to the surviving conspirators, the 'shooting range' was an underground corridor that was additionally lined with some sort of sound-proofing insulation.

Just to show how important was the testing, it should be noted that they have been limited to a single 8-shot full-auto volley using domestic ammunition. If the weapon did not malfunction, it was deemed ready to use. If it malfunctioned after firing several bullets, the test was repeated using British ammunition received from supply drops (it was usually brand new and properly stored and thus considered superior to the local one). If it still malfunctioned, it was returned for adjustment and reassembly. If the weapon malfunctioned after the first shot it was automatically returned for reassembly. It should be noted that despite such lax criteria, only 1 in 5 machine pistols were passing the test on the first attempt. Foreign weapons received from supply drops were usually not tested to minimize risk.

There is a story mentioned in some sources, although contested by others, that in September 1943, the leader of the Directorate of Diversion (unit of Land Army responsible for direct sabotage and related activities), Col. Emil Fieldorf was skeptical about the machine pistol at the first sight. Then, Eng. Ryszard Białostocki, head of the weapon department of Directorate of Conspiratorial Production, who was presenting the weapon, decided to make a public test. Assisted by several members of the Land Army, both men went in broad daylight to a nearby park, where Białostocki took the weapon he was hiding under a coat and fired several bullets into a tree, obviously causing panic. All men then escaped mingling with the fleeing crowd. After this daring demonstration of efficiency and concealability of the new weapon, Fieldorf greenlighted its production.