What you have is a ZIP gun.
What you have is a zip gun. There were guns improvised from a few items that were put together hastily in a variety of places and situations - especially in Poland.
You have to remember that many German enlisted men and officers were killed by political prisoners who were never detected as the killers, and most of those killings happened outside of the confines of the camps. These went around in groups, some of which were organized groups and some of which were improvised groups. At times, the groups fought each other, over food or over territorial issues. Many were in the partisan movements and fought with improvised weapons.
Auschwitz was a massive operation with many satelite camps. Many of the satelite camps supplied workers - many of whom were machine operators working metal projects. Quite a number of political prisoners were skilled machine operators in factories that made everything from tools to truck and place parts to weapon parts, etc, etc, etc. Not everyone who was sent to Auschwitz was placed in a killing center. Some were registered labor working on specific production goals. There were even some who made chemical components for primers and powders for small arms ammo, artillery, solid rocket propellants, etc, etc.
The workers were not always searched. In fact, weeks could go by and no searches took place. There were no metal detectors. Workers were awaken in a barracks, counted in a muster place, formed into columns and marched to their work station. At night, they were marched back to the muster place, counted, put on to barrack details and then sent in to sleep. Many weapons were manufactured. Many improvised explosives were manufactured. Many of these items were smuggled OUT as well as IN to the camps. There were partisan groups and resistance groups on the outside who needed product from the shops. There were also organized criminals operating who needed the weapons and product, often to trade.
There were also many weapons manufactured on the outside that were used in ghetto uprisings and assasinations. When political prisoners were taken, often participants in these uprisings were taken and the Germans having no idea who they had just rounded up, as was often the case. Many smuggled weapons in with them by simply bording a transport vehicle among a large group of prisoners. Most had not been able to bathe and due to this, some soldiers did not want to search them at all. Many were covered with lice, making smuggling all the easier.
Something else to keep in mind. A German NCO was also referred to as an "Officer" commonly in Europe. Those who patrolled the ghettos were often sniped at - especially at night - and many died. Often there was no way to find out who killed them. Many were stabbed to death, some beat to death, some shot, some pushed off of buildings, etc, etc. Even when patrolling in pairs, some were lost. When ghettos were liquidated, large numbers of the occupants were sent to a form of sortierungs platz - a sorting place -where they were classified as to usefulness to the Reich, based upon experience and occupation. Other sortierungs platz were to sort valuables, and not people. Many prisoners were simply not searched, or they buried items that they returned for later.
Yes, his story is entirely possible. He was obviously skilled enough to make a pistol....then he most probably had access to tools......and so was probably assigned to a production facility as a skilled laborer. Had he been old, or sick, or disabled or lacking certain needed industrial skills he would have been sent to a different division of the Auscwitz system.
Most of the Auschwitz system was captured by Russian troops, who simply set all workers free and all of the prisoners free. Many wandered the country side and many joined partisan groups. Some were too weak to leave and died of illness even after liberation. Russian forces had no food to spare, though there were some examples of Russian kindness that did take place. Many of thes prisoners later came into contact with American units who had plenty of food and rations to give and share. Sometimes prisoners traded with Americans for coats, trousers, shoes and boots especially.
There were hundreds of thousands wandering about the scene like the man you described who gave up the pistol. The story is entirely possible and probably was the truth.