Billy is correct. The 280 is a little longer to the shoulder. The .270 will fit in the .280 chamber but the .280 will not fit in the .270.
You can make the .270 brass into .280, but it would actually be better to make your .280 brass from 30-06.
Here's why: The .270 brass is shorter so it won't headspace properly in the 280 chamber. Just opening it to .280 and firing it will cause it to stretch. You will eventually get case head separation.
In order to get 270 brass to headspace in a 280, you have to put a "false shoulder" on the brass. To make a proper false shoulder, you need to open the case up even more which means using a .30-06 or .308 die. After the case is opened to .308, use the .280 die to resize the brass. You want to set your .280 sizing die to size the neck only enough for you to just just just close the bolt in your rifle. Did you get that? A .30-06 wont chamber in a .280. As you size the 30-06 neck down to 280, the case will start to chamber. Size a little more and it will go in a little more. If you look at the brass, you can see that little shoulder in the neck starting to form. Keep sizing the neck on the brass until your bolt will close. Now set your die and size the rest of your brass.
The brass will headspace properly in a .280 now and your first shot will cause the shoulder to "fireform" to your chamber. Use a mild book load load for these fireform loads. After the cases are fireformed you can then load up your .280 loads.
Anytime you make "wildcat" brass (brass is used for something different than headstamp indicates) you want to make sure you mark the brass so you will know. I paint some of the case heads black, especially if the ammo being made looks similar to the original case. I make 7.7 Jap, and one of my other 7.62 European cartriges with -06 brass. I paint the case heads black so I know they are not -06 and I make sure the ammo box has BIG writing on it to indicate the ammo inside.
Last year my two hunting buddies and I stopped at a "rest" area along a highway to our deer hunting spot. Nice lake that folks camp at and/or picnic. While we were having a sandwich, my buddies son asked if he could shoot his rifle since he had not shot it for several weeks. We were alone and it is a safe area so we said sure. The kid gets into the truck, pulls out his 300 WSM rifle and his blue ammo box. We are standing at the front of the truck eating a sandwich and he is at the back of the truck getting ready to shoot. Next thing we know he is pounding on his bolt trying to get it to close. Can't get bolt to close. We came back to check it out. My buddy tries to get the bolt to close. No luck. I pick up the ammo box. Hey, did you put one of these in your gun? Yep! He put a .280 into a 300 WSM chamber. Dad and Son both had blue ammo boxes. Neither had marked their box with the ammo name. The 280 was longer enough and smaller in diameter enough that the brass case was actually crammed into the rifling. We had to use a cleaning rod to get the round out of the chamber. First store we came to I purchase a permanent marker and marked their ammo boxes for them.
Hope this helps some. There are lots of things you can do with what you have. You just have to do it right and keep it safe.
Good luck with your ammo and make sure you mark the box!!
Steve
