That trigger looks like it would work OK in conversion uppers too since it looks like a standard AR15 trigger and hammer.
Many years ago, I bunked up an 80% lower. One of my fire control holes were off. I don't remember which, but one of them was just enough that the trigger was binding.
And to get that lower to work right, I had to shorten the disconnect top a little because the hammer was battering it when the bolt cycled. When I did and released the trigger, there wasn't enough meat for the hammer to hold on to at that point, which resulted in what I thought was a "double" at the time. It fired upon the pull, and fired again upon the release.
Later that evening, I tig welded the hole and redrilled it and discarded the mucked up parts because I didn't want to risk being in violation because of an honest screwup, but that is one of the things that I took away from that mistake. It looks like doubletap triggers has discovered essentially the same thing as I did by accident from looking at their parts assembly.
I did some work on an AR trigger years ago, trimming sear surfaces, changing angles, polishing, etc. and had it down to a very crisp 1.5lb pull. That was great, but it also made the barbaric reset that much more obvious....so off I went to work on the hammer to get the reset shorter. Worked fantastic for thousands of rounds (it's better than many of the big name triggers out there), then one day I decided to add a heavy hammer spring to it for a 5.45 upper. That's when it started to exhibit some very interesting properties.
Occasionally I would get doubles, and simply attributed it to an inadvertent bumpfire (it happens on light triggers and semi-autos every now and then). One day I was particularly concentrating on a shot and had it let go of 4 shots all at once, which is unusual because I can't bumpfire to save my life and I could feel some trigger slap to it. So I dropped the mag and got to investigating with dry fire.
The amount of force the hammer exerts on the disconnector (in the direction of restting the trigger) was more than the force required to pull the trigger. If you exerted just the right amount of pull on the trigger, it would fire, the hammer would catch and apply more forward force to the trigger, and your finger would be "reset" along with the trigger. One it resets, the forward force caused by the hammer is gone, but since you're still putting 1.5-2lbs of force on it from the last pull, your finger has no more resistance and pulls the trigger again. Apparently my finger was flapping back and forth at cyclic rate but only ever so slightly...I'd imagine high speed video would be pretty amusing. It's sort of like bumpfiring but self contained within the trigger itself, and instead of recoil forces, you're dealing with spring forces.
The trigger operated normally if you didn't ride or maintain pressure on it, and the hammer wouldn't fall if you hold the trigger down. If you don't hold any pressure on the trigger during release, it resets normally and fires normally. Quite a bit different in operation compared to a binary or a double, as technically the hammer didn't release unless you pulled it yourself...one shot per trigger pull. I would imagine if I ever did find a way to replicate such a trigger, that it would likely be given approval with ATF. Problem being, I don't know how I'd be able to "shut it off", so it would be a range toy at best.
At any rate, the spring was removed and replaced with the old one. The last thing I want is to have a second shot go off when I'm not wanting it, and didn't want to risk the possibility that ATF wouldn't find it as funny as I did.
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