Nobody wants to hear "operator error" when looking for help over a problem like this, but I think the problem is simply that your decapper / sizer die isn't correctly adjusted.
The brass isn't the problem. The last .270 I loaded last month got loaded into once-fired Federal and unfired, nickle plated R-P. The bullets aren't the problem. I used Hornady bullets, too. Measured .277" like I knew they would. Your case cleaning and lube aren't the problem. Your bullet seater is doing exactly what a properly made and improperly adjusted bullet seater should do.
But in adjusting the bullet seater to get crimp to solve a problem that you shouldn't have, you're adjusting the wrong die!!!
Did that set of dies come with instructions for adjusting the decapper / sizer die? If so, did they say something like "Proceed by inserting the correct shell holder, indicated on the die box label, into your press ram..." And did you do that?
The protocall for adjusting a sizer / decapper isn't uniform from one brand to the next. Neither are the height of shell holders. One thing that is pretty common is that instructions for adjustment almost always involve indexing off a ram at full upward travel, with the "correct" shell holder inserted, and the die screwed down to make contact with it.
Your expander ball is fine at .2755. You only need an interference fit of .001" less than bullet diameter for adequate neck tension. And .002" less is about the most you ever want or are likely to get. You don't want it lower than that because then you'll be resizing the case again during the bullet seating operation and working the brass again when you don't really need to.
Let's think about how this thing works for a minute. Your fired case has a neck opening at least .277" or there would still be a bullet in it. So when you run that up into the die, what effect does the expander ball of .0015" smaller than what is supposed to expand really have? None, right? Because on the upstroke, there is nothing to expand. If your die is correctly adjusted, the neck will get squeezed on the upstroke to a dimension smaller than the expander ball. When you lower the ram, the expander ball gives the neck its finished interior dimension. That should measure .001" to .002" less than bullet diameter. That's all you need for a good interference fit and proper neck tension and for reasons to detailed to get in to here, that's all you really want.
So if this isn't happening, odds are that your case isn't getting pushed up far enough into the die for any neck sizing to occur. If you adjust a sizer die too deep, you get a sized neck but a shoulder pushed so far back that it causes the body below it to buckle. So we know what direction we need to screw your sizer die....
What I would do is break out my caliper, lube a case, run the lock ring up the die and out of the way, screw the die in an eighth of a turn, run the case through it, and measure the neck interior. I'd keep doing that an eighth at a time until I got to the point where I was neck sizing the case and only barely partially resizing the body and I'd stop when my measurement read .276" .2755" and load away. When my cases started getting a little tough to chamber, I'd screw the die in another eighth, run all my cases through that to bump their shoulder back, then back the die out the eighth I screwed it in, and keep partial-sizing until the cases were recyle material.
JP
