Changing the bolt carrier group doesn't fix it. With no bolt in, the upper opens and closes with ease. Install the bolt, and you have to slam it to get it to open and close. Thoughts? Suggestions?
Evidently the bolt carrier is not moving forward enough.
Have you cleaned/lubed the bolt/carrier?
A very stiff, or dry, bolt might cause this issue.
Good luck
if changing the BCG doesn't change anything, the only other thing I can think of is upper out of spec. Can you see any marks inside the upper where the bcg is hanging up?
No marks. Slamming the forward assist doesn't budge the bcg. It looks like it's as tight as it can go. I'm leaning towards either barrel or upper is out of spec, but I have no idea how to figure that out.
with the bolt removed, you could cover the end of the bcg or barrel with tape, install the BCG & see if the tape is crushed. I would think that would validate or invalidate the barrel being out of spec.
Or if you have a caliper with long enough rod you could check depth from the back of the upper to the barrel & compare against a known good one.
Try a different carrier. If that doesn't work, try a different gas tube. You can put the carrier in and turn the upper upside down and watch it slide from underneath. Watch and see how well the gas tube enters the key
If you have a good set of calipers, measure the OAL of the carrier. It should be no more than 6.875".
Remove the bolt and drop the carrier into the receiver. You shouldn't feel the carrier key interact with the gas tube on the way forward. If the gas tube is binding in the key this can cause your issue as others have mentioned. Could be a bent or improperly attached key. I've actually seen more of this on cheaper BCGs than I've seen bent gas tubes.
ETA: this could also be caused by the hole for the gas tube to pass through the upper being off location. If the gas tube is binding, try checking that out by inserting a pin gauge (or the smooth side of a tightly fitting drill bit) into the carrier key and check alignment by inserting the carrier into the upper slowly.
Oh man...I don't want to hurt any feels but are you married to SanTan? Perhaps get that money back? I'm no expert and I know there's countless manufacturers out there but dang man. We should all be able to build a frankengun without concerns like these. Either way, glad you found the problem, good luck on the replacement.