Re: Bullet bearing surface question
What you're running into is normal and not a bad thing really.
No doubt the 277 gizzy is drilled to ~.271, which correlates with bore ID rather than groove. A 284 would likely be drilled to ~.277(it's bore ID), which is why it mates very closely to 277 bearing begin.
It depends on the bullet diameters(which vary with brand and lot to lot) as to how consistantly you can measure. The closer you get to actual bearing, the tougher it is to get consistant measurements because slight changes in contact pressure start affecting the readings.
You might use 284 gizzys front and back, if your bearings are large enough and you control contact pressure with a bit of precision. This wouldn't be something done for damn with calipers.. You would need a Tubbs/Buhay comparator.