there was also a device I've seen at a gun show a few years back that essentially helps you bump fire the gun. Claimed it was 100% legal. Didn't care too much either way, it's a waste of ammo.
The lighting link is only for AR's and is an NFA item (around $7500 + $200 stamp and 6 month wait)
Beyond that there are a lot of ways to get into trouble. There have been a few that were initially approved by the BATFE, AW-SIM and Akins Accelerator come to mind first but when it turned out that they worked as advertised the approval was rescinded.
This should give you a good idea of the quality of the products that remain legal.