Bob Stewart advertised the recoil as "less than a twelve-gauge shotgun".
Whatever. The point is, you're implying that these kits are not safe for even a professional to modify and fire. Is that what you'll have me believe?
Where did you read that the ATF did not fire a live round and that if they did it would be fatal?
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?