16)
I installed an HK upper on my non-HK lower, and the gun won’t fire!
To reduce the risk of slam-firing, HK installed a firing pin safety. To disengage the safety, you need a mil-spec style hammer without the “cut-out” at the top. Otherwise, the safety will not disengage, and the firing pin will not be struck by the hammer; thus, it won’t fire.
http://i138.photobucket.com/albums/q...445/hammer.png
17)
What if I remove the firing pin safety?
It's recommend this as I have mentioned the increased bolt speed and risk of slam firing (especially with primers that are improperly seated).
18)
What about match triggers?
Again, a certain amount of hammer speed is required for reliable primer ignition while disengaging the firing pin safety and acting against the firing pin (not found in standard AR's). Some are not built to withstand the increased force/speeds associated with this system over time. Also you need to make sure the hammer is the appropriate height.
Montrala has formed a comprehensive list of triggers that have been tried in HK guns
