The GAT trigger is the only one that is made with any kind of quality. The others you could make yourself with crap from the local hardware store, and it would work about as well, namely not at all.
Most of them are just a way to automate so-called "bump-firing", which you can do from the hip yourself without hanging any extra doohickies off your gun. Other than that, my advice is, put the gun on your shoulder and learn to pull the trigger FAST in semi-auto.
Case in point: me with my old .45acp AR15 setup, long ago sold. This is with a factory Bushmaster trigger and no modifications. Just fast fingers is all. How much faster do you want to go?
www.imagestation.com/video/view.html?id=4253377719&dl=1www.imagestation.com/video/view.html?id=4253377916&dl=1Hit right-click, "save as..." on these to get the links to work. The .45acp mags only held 15 but believe me I can do it with 30, 40, or even 75 if I want to.
P.S. Nothing says "why yes, I still live in a double-wide trailer with my parents" like an SKS in a plastic stock with a jury-rigged 30-round "clip" that doesn't really attach or detach from the gun without welding and/or dremeling and an old stamped steel Hellfire trigger system rusting next to the trigger. Don't be that person.