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Posted: 8/5/2016 9:46:52 AM EDT
have a friend who just built a couple of carbines, took them out to fire, one functions with no apparent problems, the other, upon firing the first round, it started to do a "double tap". he'd fire a round, it would cycle, pick up and fire the second round, fire and then revert back to semi-auto mode. both weapons are pretty much identical with the exception of the lower by name brand. all internals appear to be correctly installed. any thoughts of wisdom? thanks, joe g.
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Doulble tap is a problem with the disconnector not holding the hammer back correctly, hence hammer held back on the disco until the trigger is completely released, and the trigger front sear in the needed postion to catch the hammer from trigger/disco release.
So step one is to make sure that the correct disco spring was installed in the first place, and it was installed with it large spring coil side down into the trigger rear slot under the disco tail. 
Also, need to check to make sure that the hammer spring was installed in the correct orientation, and as well as the rest of the springs. 
Now once the springs have been confirmed install correctly, step two is to check the free gap that the back hammer sear clears past the disco sear when the hammer is cocked back even with the two sears as close as they can get to each other, while not touching the trigger. The free gap that is needed for the two to pass each other is .001 to .003" (the thickness of a hair). Also, with the hammer cocked all the way back against the top of the disco tail, confirm that the tail of the hammer is only making contact to the tail of the disco, and no part of the rounded hook back surface just before the tail of the disco instead. In this photo, the free gap is a tad too wide to allow a reliable disco reset on trigger release, 
While the free gap in the below is way too wide isntead, 
Now if the disco to rear hammer sear free gap is too wide, start by removing the disco to make sure that there is no debris between the front/bottom of the disco and the top/front of the hammer it riding on,that the bottom front of the disco does not have a bur that may not be allowing it set all the way down on the top of the trigger. Also, make sure that when the hammer and disco are free fitted out side the receiver with the pin, that the disco is moving freely in the trigger slot (just the disco spring forcing it back forward against the front of the trigger. In this photo, we can see that the front surface of the disco where it going to seat against the hammer is less than stellar, and little clean up to square the surface should solve any free gap problem is has, 
On other cheap kits, the bottom front pad of the disco is so stamped crated, that by the time you try to clean up the bottom contact pad surface, you would be in negitive free gap isntead. If all the above are clean, then either the disco will need to be replaced, or will need to be re-timed instead. The short version of re-timing a disco to bring it back in spec, is to remove metal from the bottom/front of it where it will seat against the top/front of the trigger, to bring the free gap into the .001 to .003" range. 
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