Double fire - no sear engagement solved.
Thought this could help some people.
I had major issues with a tantal - fail to fire and all the primers were smacked hard, sometimes it would double fire.
turns out my sear was not catching the hammer every time. This is because i put the trigger too low hoping for a super sensitive light trigger - sear was so low it would not always catch the hammer. Instead it would simply butt into it.
Solution, grind a mm or so off the sear hook surface effectively elevating the sear enough to grab the hammer every time.
No more double fires, no more sear not grabbing the hammer.