The National Firearms Act and Weapons Configuration Legality:
The regulations from the National Firearms Act (NFA) regarding barrel length and the legality of adding a rifle stock and forward pistol grip are very confusing. What constitutes a “short barrel rifle” (SBR), a “short barrel shotgun” (SBS), or Any Other Weapon (AOW) and requires a $200 tax stamp from the BATFE? With all of the cool and useful firearms parts and accessories available it could be very easy to illegally configure your pistol, rifle, or shotgun. I found these diagrams on the internet that concisely break down and simplify the issue.
Shotguns:
AK Style Rifles & Pistols:
HK 91/93/94 Style Rifles & Pistols:
The Savannah Arsenal Project 