As an expert in the field of legal studies with a focus on firearms legislation, I can provide a comprehensive answer to your question regarding the legality of converting semi-automatic firearms to fully automatic. It's important to note that laws can vary significantly by jurisdiction, and the following answer is based on United States federal law, which may not apply to all states or other countries.
Step 1: English Answer
The legality of converting a semi-automatic firearm to a fully automatic one is a complex issue that is regulated by both federal and state laws in the United States. The primary piece of legislation governing this issue is the **Firearm Owners' Protection Act (FOPA)** of 1986, which imposed significant restrictions on the civilian ownership of fully automatic firearms.

Under the FOPA, fully automatic weapons became technically legal only if they were made before 1986. This means that any fully automatic firearm manufactured after this cutoff date cannot be legally owned by a civilian in the United States, unless it falls under a specific exemption, such as being owned by a licensed collector or used for certain law enforcement purposes.

However, the situation becomes more nuanced when it comes to the conversion of semi-automatic firearms to fully automatic. The sale and possession of modification kits or parts that can convert a semi-automatic firearm into a fully automatic one are also regulated. While it is illegal to manufacture or import such conversion kits, there is a loophole in the law that has been the subject of debate. This loophole pertains to the sale and purchase of these kits, which, depending on the interpretation and enforcement, can sometimes be legally sold and bought.

It is crucial to understand that even if a modification kit is legally sold, the actual act of converting a semi-automatic firearm to a fully automatic one without the proper licensing and registration is illegal. The Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) strictly regulates the conversion of firearms and requires that any fully automatic firearm be registered as such in the National Firearms Registration and Transfer Record (NFRTR).

Furthermore, individuals who wish to legally possess a fully automatic firearm must go through a rigorous process that includes a thorough background check, fingerprinting, and payment of a special tax. The process is highly regulated and can be quite costly and time-consuming.

State laws can also impact the legality of such conversions. Some states have stricter regulations than the federal government and may outright ban the possession or conversion of semi-automatic firearms to fully automatic, regardless of the age of the weapon or the legality of the conversion kit.

In conclusion, while there are certain circumstances under which fully automatic weapons may be legally owned, the conversion of semi-automatic firearms to fully automatic is heavily regulated and generally illegal without strict adherence to federal and state laws. It is always advisable to consult with a legal expert or the ATF for the most current and accurate information regarding firearms law.

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Under federal law, fully automatic weapons are technically legal only if made before 1986, when Congress passed the Firearm Owners' Protection Act. ... The second major loophole is that it's legal to sell and buy modification kits that can convert semiautomatic weapons into effectively automatic ones.Oct 4, 2017read more >>