I thought AR-15 receivers had a difference in machining that prevented an auto sear from being installed.
CC
Watch the video at the article.
The youtuber involved in the case explains pretty clearly how the carrier group is the part that is modified to make the regular AR in to full auto.
Colt made their bolt carrier more difficult to modify.
They don't fit all commercial lower receivers (which is why the article specified "some AR's") but those can be milled out slightly to make them fit. That isn't "all" you need to give the gun select fire capability, though - a three-position fire selector switch is necessary as well, which some lowers won't accommodate. And as someone else already pointed out, the bolt carrier group may or may not be full auto capable, although the ones that are, are Milspec and fairly common. These are often advertised as "AR-15/M-16" BCGs.
This all sounds easier than it is - you don't just cut the sear out of a card with tin snips, it has to be machined within spec. There are other alternatives: there are drop-in trigger groups that are full auto capable, which are Class 3 parts and already controlled under the NFA. There are forced-reset trigger groups that are currently being vigorously prosecuted by the ATF although whether they are full-auto by definition is still controversial. There are, as well, binary triggers that shoot once when pulled and once upon release that are not technically full auto but are under close scrutiny by the ATF as if they are. If you can pull a trigger five times in a second, that's ten shots a second, which is roughly the full-auto cyclical rate of that platform anyway. The ATF has tried to expand the ban to devices that allow the firearm to be shot at a full-auto rate but that has proven unacceptably vague - after all, although I don't know anyone who can, in theory if you can pull the trigger ten times in a second your finger would become a Class 3 part under that definition. The Supreme Court has already ruled against the bump stock ban on that basis if I understand the matter correctly, although I am not an attorney.
I'd love to see the NFA overturned because applying it to suppressors and "short-barreled" rifles is silly, IMHO, but I doubt it's going to happen. Just my $0.02.
There are “high shelf” and “low shelf” lower receivers. A “low shelf” will accept the auto seer without need to make any modifications to the lower. A “high shelf” lower can be further milled out to accept the auto seer, but it’s a lot easier to just find a “low shelf” lower.