BudsGunShop.com - Why the long barrel?

Logan, since this is a Pistol Caliber Carbine, it has to have a minimum of a 16 inch barrel to be classified as a Rifle/Carbine. Shorter than a 16 inch barrel would make it classified as a SBR (Short Barreled Rifle) and with a shoulder stock would need to be registered as an NFA Firearm and a $200 Tax Stamp paid.

The barrel is only as long as it needs to be. Rifles have a minimum length of 16 inches before being considered a SBR that needs a tax stamp. Not sure if Thompson makes one without a buttstock and shorter barrel, that would be marketed as a pistol.

This gun is CLASSIFIED as a rifle and must have a minimum barrel length of 16 inches to NOT be considered as a Short Barrel Rifle (SBR). SBR requires additional stamp and approval from ATF to own or purchase. I hope this helps.

the barrel lenght restrictions only apply because it has a stock if one were to remove the stock and replace it with a stabilizing brace ypu could have any lenght barrel on that you wanted, without having to go through the atf to classify it as an SBR, because o ce the stock is removed and the barrel is short it is no longer a rifle but a pistol.
The only time you'd have to SBR It is if the it has a stock and a barrel shorter the 16" or in it has any type of vertical forgrip forward of the magazine.
Longer barrels can also be beneficial in that they give you a longer site radius making it easier to shoot accurately, and the longer travel down a rifled barrel will increase muzzle velocity, resulting in higher ft/lb force on impact, and a straighter, closer groups, and lower recoil.