Any large enough piece of steel can be machined to make anything made out of steel.
The parts he sells can be so machined.
Easily converted is not a technicality. It is the substance of the law.
Lets get this right. The fully automatic M-16 rifles have different size pins that hold the automatic capable sear. This is done so that 1. you can tell right away from the outside if it is automatic, and 2. so that automatic sears do not fit into the semiautomatic AR-15 and clones.
The ATF had a special AR-15 made up that was fully automatic, but had the semiautomatic pins. The very existance of that rifle was illegal. They took it to Stewart, and claimed, falsely, that it had a problem. From the outside it looked like a semiautomatic AR 15. When he accepted it, to work on it, they arrested him.
The ATF brought an illegal machine gun to Stewart. Then they charged him for posessing an illegal machinegun. That made him a felon.
Then they tested his kits, using the standard of "can it fire a primer?" to assert that he was a felon in posession of kits. Heck, a nail and a hammer can fire primers. Heck of a standard there.
Then the used that prosecution to go through locked storage in his home/office, that he had not sold. For what it is worth, it is a lot easier to make a fully automatic rifle than a semiautomatic rifle. These one of a kind prototypes would have been offered to the US government for sale (the largest purchaser of machineguns in the hemisphere) but needed to be developed to have a reliable and compliant semiautomatic capability. Until they were legal, they were not for sale, and were not in commerce of any kind.
This doesn't bother you? I guess you would have arrested John Browning too.
He was already a felon by that time -- he was arrested, tried, and convicted six years earlier on a machine gun possession.