Point one- ATF control over a 50 cal upper- agree with that idea, the AFTE is deep state to the hilt, Trump has not dealt with them as needed. The entire DOJ is rogue in my opinion.
Point two- the AR lower bears no recoil impulse in regards to structural requirements. The buffer tube/receiver extension and the upper rear face transfer all recoil, the take down pins simply hold the lower to the upper.
Using a decent brake ( witch in a light weight 50 cal is a given) felt/free recoil is not much more than a 12 gag 3 inch mag load- maybe 45-50 ftlb, less than a 416 Rigby or 458 Win in a similar 8-9 lb rifle.
a 416 or 458 is WAY more than 45-50 ft lb in an 8-9 pound rifle! a 416 with heavy loads is closer to 80 in a 9 or 10 pounder. A 50 BMG at 30 pounds will be less, I’ll give you that one.