The ATF in court asserted that they had detonated a primer inside an empty case.
They also asserted that was all they had to do to demonstrate that that the thing they had so altered was a firearm.
When fired, the recoil of the Madii Griffin was sightly less than that of a 12 gauge, measured at the butt. The forces inside the rifle would be much larger than that for a .50BMG round. Stewart's rifle uses an efficient muzzle brake that uses burned gas to reduce the recoil. (same approach in general as the Armalite .50, the Ronnie Barrett's M-107 and the Windrunner .50BMG rifles.
As for being safe to modify and fire, they are (I have fired one) but not using a dremel tool as per ATF testimony. Rather you would have to use a milling machine or lathe to accurately machine the parts. You sure aren't going to do it in 30 minutes.
Is is starting to get through how the ATF changed meaning of .50BMG rifle so they could get the rifle to pass the silly test they invented?
So you concede that the parts can be machined to produce a working .50BMG? So, he is indeed breaking the law, but you believe that he should get off on a technicality -- ie., depending on the definition of "easily converted".
Now, you say this conversion is complicated and would take some time, thereby negating the "easily converted" criteria of the law. Well, first of all, the district court found that the affidavit, which contained Stewart’s own statements about how easily his kits could be converted, still supported a finding of probable cause. Let me guess, Stewart was lying.
Second, if this went to court, maybe the judge would have found that it wasn't so easy. But Stewart was never charged with selling these kits. This was never going to court. This is a non-issue!