I am still researching this whole thing, 1. The safety of the device is the first factor and from what I have read the second shot can be negated by simply turning the selector off the third position resetting the trigger back to semi only. 2. The legality with the ATF, The ATF has provided a letter saying its legal. 3. Is it allowed in competition? The desire is for a faster double tap.
This video shows the negation of the second round.
The desire is of course for a faster double tap BUT not at the cost of safety or running afowl of the rules
IPSC GUY