There are certain firearms and accessories for them that are regulated by the National Firearms Act in 1934, including machine guns, suppressors, short barreled weapons, and other “highly dangerous” items. However, these items are restricted and not completely illegal. The only barrier to legally owning any of these regulated items is a moderate sum of cash, both for the rare and expensive items and the ATF tax stamp that usually costs $200. Not the most economical option, and it certainly may not be possible for the average gun owner to afford most NFA items, but the most commonly dealt with restricted weapons and attachments are Short Barreled Rifles and Suppressors.

As per the National Firearms Act, the minimum legal length for a rifle barrel is 16″ measured from breech to the end of the barrel or fixed muzzle device. For shotguns, this minimum length is 18″.
If your weapon has a fixed or folding stock and a barrel less than the minimum length, it is considered a short barreled weapon and must be registered and have a tax stamp paid to the ATF. This is done by submitting an ATF Form 4 and the $200 tax stamp either online or through the mail and waiting the 1-6 months to receive approval. The short barrel weapon may not be assembled in any way until the approval is obtained through an FFL with a Class 3 SOT.

The other most popular NFA items are Suppressors, also known as Silencers. These were included in the original 1934 NFA, and still continue to be a difficult item to get a hold of despite them being simple attachments and not actual weapons. However, the same process applies with submitting an ATF Form 4 and $200 Tax Stamp. After the forms are sent to the ATF and are pending approval, the suppressor may be shipped to the FFL but they cannot be transferred until the approval is granted.

Although you may have been led to believe that machine guns are illegal to own, the truth is that there is a way for an ordinary citizen to own a fully automatic and legal weapon. As mentioned earlier, however, these come at an extremely high cost, over 10x as much as SBRs and Suppressors.