The friction can be due to the upper being a bit tight inside, or it could be the stock is slightly misaligned. I'd remove the stock and see how the BCG moves just to make sure. If it's still tight, than it's just the upper.
If the stock is aligned, then the internal buffer tube won't be the source of friction. It has a lot of clearance.
I've found that a generous amount of lithium grease makes a huge difference, even without any sanding.
