Aaaaaaaaaarrrrrrrrrrrgggggggggghhhhhhhhhh hate this crap lipos are batteries not bombs. Use a smart charger and dont pierce them and they are fine. Your phone battery is more dangerous if it goes bang and you are more likely to burn your house down cooking toast than you are charging a lipo.
Samarai could be correct because it should fire ok if everything was replaced correctly I would tend to think.....
But if your Krytac was shooting at 420 out of box then it probably wasn't a UK gun so it could have the faster 30k motor not 20k motor in there
Then add in you are running 11.1v on a m100 spring with neodym you will be hitting 25-30rps and yes at this speed with m100 it will double fire
(well quite likely you are going to be 30 or over I would think on good neodym & 11.1v)
Double fire I mean it will cycle twice on semi
(God knows how many times people say double fire when in fact it isn't double firing but friggin' misfeeding and nothing then 2 then 1 bb's come out)
So by what you said on that much juice it quite likely to fire twice on semi......
Your basic options is to drop to 7.4v or 9.6v and stick in low to mid 20's
or
old spring back in and ShortStroke the bloody thing to reduce fps aprox 15 to 20 fps pre tooth removal
but depending on barrel length you may only be able to remove 2 to 4 teeth
of course you could put in a m110 and just shortstroke 1 or 2 teeth if needed
or fit a less powerful motor to prevent it going nutz and firing twice on m100 spring
should still fire ok on semi though unless you did something wrong or it really going mental in which case PE may occur if you continue
the double firing on semi is an early warning sign of you taking the pi$$ so it needs to brought under control a bit
a little pre-cocking is ok but not firing twice - that is going too quick on present setup imho