IMO "drop in" means that a 'hobbyist' can install it with minimal effort and simple hand tools, as opposed to a "gunsmith/professional fit". As a gunsmith I rarely see drop in parts that drop in. They have to account for variations in manufacturing and between models, so the parts are usually a bit over sized. It has been a while since I installed a ghost trigger, but I believe there is a tab on the bar that trips the sear that needs to be bent or filed down to fit. Should say in the instructions that came with it where to remove metal so the trigger will move far enough to trip the sear and release the striker. Don't try to remove a lot of material at once, take baby steps. If you go too far you will wind up with over travel. A couple swipes with a file, assemble, and function check. Repeat if function check failed. Unfortunately you have to do the back plate trick willfully armed mentioned earlier each time to disassemble it until you get it working. It is a pretty simple process, just go slow and carefully. The ghost triggers are real nice once they are fit.
Scheme well, scheme often.
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