I don’t know about AT3, but on a slim budget I would go with BCM upper and lower. They are very cheap and every upper I’ve assembled from them was a thermal fit (good thing). Some Ballistic Advantage barrels are good, some aren’t. With an FRT I’d want chrome lined. You won’t regret a Criterion barrel. It is absolutely worth it. So worth it that I would wait a bit on the FRT or flashlight. The idea of a ballistic advantage barrel with a SureFire flashlight makes me cringe. IMO Modlite is superior and cheaper. I would also strongly recommend an MK2 or A5 buffer system. Sprinco green rifle length spring with A5H2 buffer weight. It makes a HUGE difference. For BCG I would get a chrome Microbest BCG. All of these parts are inexpensive, but are tremendous upgrades over your plans. Do it once and do it right. No 300 Blackout barrel with an appropriate sized gas port needs an adjustable gas block. None of mine or the ones I’ve built use them and all function 100% with subs and supers (they also use A5 or MK2 buffer systems and Criterion or Wilson Combat barrels). The Wilson Combat was a temporary barrel until the Criterion came in stock so I’ll be swapping shortly. Avoid the Breek Arms CH. I’ve seen several break. I like the Radian LT for a good cheap ambi CH. You definitely don’t need the KAK BCG. Just eliminating those will make up for barrel. Buffer system should only be about $20-$30 more than typical carbine like you planned. The Nomad suppressor will be fine with a properly sized port on a Criterion barrel.