Hello,
  I just installed a set of JP yellow springs(hammer and trigger) and man, what a difference!!!  Best 10 bucks you'll ever spend on a trigger, thats for sure, as long as they fire off all ammunition. My question is, has anyone experienced any FTF because the hammer didn't have enough strength? They mention that it's possible in the directions, but it seems to have plenty of snap to me. As my upper is not yet assembled, I have not had a chance to fire the weapon, but that ominous warning in the directions is a bit un nerving. Thanks in advance!!!  

I have these springs in two of my rigs, they're very nice, and I've had no issues of any kind.

Yes, I have.  The yellow spring will sometimes not fire hard primers.  Its works fine with commercial spec ammo, but mil spec hard primers may not fire.  They all fired when struck a second time.

I have the yellow springs (along with the JP trigger & hammer) in one of my ARs and the only FTFs have been with a Ciener .22LR kit installed.  About 90% failures with it, but none with .223s.  That includes Q3131A, Wolf, and plenty of other .223 loads.

I had to take mine out of both my fresh builds as the FTF were constant.

The guys at JP told me the key to reliability with the yellow springs is having a speed hammer installed. The tab on a stock hammer can be ground off if you don't want to but JP's hammer. Even then, they recommend a stock hammer spring for duty use. That being said, my JP trigger/speed hammer combo has been reliable with everything so far using the reduced hammer spring.

I did a trigger job on my latest build and installed the following components:

1. JP reduced power spring kit.

2. KNS anti rotational fire control pins.

I've shot about 100 rds of LC XM193 and haven't had one problem. I guess I'll see what happens after another 900rds for a true test.

Semper-Fi Do or Die, DD out!!