Most nations have more intensive control over gun manufacturing than the US. This article describes firearms that we might consider “home-made” going from simple zip-guns and slam-fire shotguns up to mortars and recoilless rifles.
Safety“The operation of improvised or craft-produced small arms and light weapons—as opposed to professionally-manufactured weapons—often poses signiﬁcantly higher safety risks for users and bystanders. As discussed above, individual classes and types of weapons can suffer from shortcomings that range from the poor quality and consistency of raw materials (including improper material selection, heat treating, and ﬁnishing) to insufﬁcient manufacturing tolerances and a lack of quality control or proof-testing, all of which result in dubious chamber, barrel, and action integrity. In turn, these weaknesses greatly increase the risk of catastrophic failure, in which case the weapon is rendered useless and the ﬁrer may be injured or killed. If ammunition has been hand-reloaded or wholly or partly improvised, further problems may arise from too much or too little propellant, under- or over-calibre projectiles, insensitive or over-sensitive primers, and other factors.
Quite apart from these endemic problems with improvised and craft-produced ﬁrearms,controlling for correct cartridge headspace is another serious issue for designs that require the use of high-pressure conventional ammunition. Even if a weapon is otherwise well made, insufﬁcient headspace will prevent reliable feeding, and excessive headspace can result in catastrophic weapon failure and harm to the user (Ferguson, 2015).
In view of these factors, craft-produced and improvised ﬁrearms tend to be chambered for relatively low-pressure cartridges, such as .22 Long Riﬂe, .380 ACP, and shotgun cartridges. Similarly, improvised and craft-produced light weapons tend to ﬁre low-velocity projectiles or rockets, which do not exert the same pressures on a weapon system as many conventional artillery or support-weapon projectiles. However, there are other, less dramatic dangers inherent in the use of improvised or craft-produced weapons under combat conditions. Weapons need not directly harm users to put their personal safety at risk. Given that poor-quality magazines, especially magazine springs, and crudely manufactured magazine wells and feed ramps are commonplace, employing certain models of improvised and craft-produced weapons increases the risk of stoppages or failures-to-feed signiﬁcantly. Poorly manufactured critical components, such as ﬁring pins—which may be either too brittle or too malleable—can cause a weapon to experience a failure-to-ﬁre. While not as disastrous as a catastrophic failure, these problems can still expose the user to signiﬁcant danger during armed conﬂict.
These shortcomings are also likely to reduce an individual user’s or group’s conﬁdence in a weapon, which can limit its effectiveness even before any malfunction presents itself. This type of issue seriously affected the British Army in 1991, when UK troops deployed to the Gulf War in what was termed Operation Granby. Platoon commanders expected to sustain extra casualties during fortiﬁcation-clearance operations, specifically because infantrymen lacked conﬁdence in their stoppage-prone weapon systems (LANDSET, 1991, as cited in Raw, 2003). Given that low levels of conﬁdence in the reliability of weapons can have a serious impact on trained soldiers, the effects on untrained or poorly trained insurgents and armed criminals are also likely to be substantial, especially among those operating in urban areas or close terrain. Consequently, most unauthorized users prefer factory-built weapons to their improvised and craftproduced counterparts.”
The report is worth reading.