Glenn - your "fitted neck" situation has your 7.7 project paralleling what I do with .30-30. Larry's advice to size down some 8 mm bullets is what I'd go with. The gas checks certainly stay on the bullets I size down from .322" to 312".
The one thing I would add is that when you size down the 8 mm bullets you may get a dimple in the gas check. That's OK, but it may be off centre and not round. That depends on the sprue. Even if it's just the lip of the crater from the sprue cut it's worth gently shaving the bases with a sharp knife - you're not trying to get to clean metal all over, just take the high spots and get it flat. Residual pits are OK. With the bases shaved flat the dimple with be round and centred.
In reality the clean base may not make all that much difference, but given the time and effort spent on the rest of the operation It would enhance satisfaction (mine anyway) at having done the job properly.
On the subject of lumpy sprues, we hear often enough that any damage to the base of the bullet must be detrimental. I think this should really be much more about the circumference of the base than about the central portion. For example, the article by Nils Kvale (Norma Ballistician at the time) in American Rifleman compared tangential filing at an angle on the nose of the bullet (little effect) with the same on the base (achieved an effect). It did not look at damage elsewhere on the base, only on the corner. Since all (?) the air drag is on the circumference, leaving a vacuum in the center of the base, I do wonder if damage in the central part of the base is as important as we think it is. Hence the last sentence of the previous paragraph.
You are only as good as your library.
