If you cut an A2 barrel down, you have to open the gas port up. If you cut an A1 barrel down, you don't. It has to do with the thickness of the barrel at the gas port. Generally, A1's run fine that way.
Shortening the barrel forward of the gas port dorks up the 'duration'; that is, the amount of time that gas pressure is available to cycle the weapon. For whatever mystical reasons, the A1's don't seem to be plagued by this problem.
What tends to happen, is that to compensate for inadequate duration, extra volume is introduced to the system with the bigger gas hole (non-A1 models). This tends to keep the pressures high in the chamber while the weapon is attempting to extract. This in turn leads to extraction issues (duh). This is similar to the problems people have trying to get shorties to run with cheese-dick barrels.
Perhaps you've seen the 'pig tail' gas tubes that wrap around the barrel. These are designed to delay the gas heading to the bolt carrier to give the gas pressure in the fired cartridge time to drop. Going to a heavier buffer will also help.
The only thing fulton armory is 'wrong' about, is how much they are charging for those A1's with the barrel cut down.