Thanks, I really like how it turned out but I'd feel way better once it's more secured!
I don't know if staking the castle nut would work. I'm not an engineer but I think the buffer tube could potentially unscrew even with staking since it would just rotate within the adapter threads and the locked in castle nut. I think the set screw would be a more secure option, I just need to figure out if I can do that myself. Will update again after I try it.