on some, at the time, it was as simple as DRILLING an extra hole and putting in a pin so the change lever couldn't go all the way over- on my uzi, they changed the design of the lever itself so there's a "step" that doesn't allow it to go to f/a fire- on the m14, they had part of the selector switch ground off and welded inoperable- it varies widely- and if memory serves, you had to have it "certified" that the work was done properly - that meant going to a smith, not bubba- and when all was said and done, they changed it yet again- this time they demanded a non-removable trigger group, which meant welding in the takedown pin, and a trip off to rcmp lab for certification