Question that I think I know the answer to:

I am looking at the reloading data (Barnes in this case) for a 280 vs. a 270 and find that for the same bullet weight with the same barrel length, that the manual suggests 1.0 to 4.0 fewer grains of the same powder in the .280 and gets from 100-200 less fps.
Okay, is this because the 280 is under loaded for fear of damaging a Remington pump or auto? If so and if all other things are more or less equal, since the case is virtually the same size and the pressure with the same bullet weight should be slightly less given there would be less because of less contact area, why couldn't I use .270 data in my .280 (a rechambered Ruger Number One)and expect the same or better speed?

Well,hell I posted this on the wrong group. Maybe some of you varmint hunters know the answer. Sorry...

In theory you are right for two reasons:

1) The 280 has greater case capacity than the 270.

2) The 280 has a larger bullet base area.

So in theory that will give you with equal charges of powder and equal bullet weight less pressure.

But...

Just as I would not take anyone's "pet" 270 load and use it in MY 270 without reducing at least 5% and working up so I wouldn't with transposing 270 information for use in a 280.

Again, in theory, you can for the same reason as in 1) also directly take 7x64 information and use it in a 280 Remington.

But as with all handloading that "reduce 5% and work up" is sound safety parctice.

I have BOTH a 270 and a 280 and if you look at those spiral bound Reloading Notebook publications you will see (on the rare times when a 150 grain load is listed - or with 140 grain bullets) that for the 280 the powder charge equals of exceeds that of the 270.

FOR WHEN USING A MODERN BOLT ACTION RIFLE OTHER THAN A PUMP ACTION OR SLIDE ACTION....

You are correct in your assumption that most of the data listed for the 280 is at a lower pressure reading than the 270. You CAN use any data that you want to in your own rifle, but getting someone else to endorse it as a safe practice is probably an entirely different matter. Speaking in general terms the larger the bore the less pressure it takes to move the same weight/shape bullet at the same speed.
You might need to expand your search for a loading manual that does load the 280 to its full potential.