The origional had primer modules and there were problems with fitting these modules,, the ATF guys also considered the origional a regular firearm so watch recieving one without a ffl involved
The ml2 has no priming module, a better design with a breechplug that the bolt holds a primer and holds it in place and also pulls it out after firing, not considered a regular firearm no FFL needed
ML2 is a much better system
