I ordered a new 280AI a couple weeks ago and it will be until June before it gets here. I have already gotten in my reloading components, dies, brass, bullets etc. I would like to have some rounds ready when I get the rifle. Would you guys go ahead and load a bunch now based on SAAMI OACL and load to the max or would you wait and measure the rifle chamber before you load so that I could load the bullets .020 inches off the lands? I know what to do once I have some once fired brass.

For sure wait.

Its tuff to do it but what he said ^^^^

IMO, no problem loading enough to get through a breakin, maybe 20 rounds, Run your brass over the sizer button to make sure its perfect round,

On second thought, I would only do this if I had some factory rounds loaded with the same bullet you are going to load. Set your seater to the factory round. Otherwise you might have to pull bullets or seat them deeper.

I'm in the same boat, Having a custom built myself, ready about the same time yours is. I want to load some test loads but without making sure the throat is perfect (no neck turning needed) and measure each bullet type to the lands could be causing a huge waste of time pulling bullets.

try to be patient

Load a few to get going. No harm in that. That said, I've pulled a few bullets in my time, even after I had the gun.

How are you going to know if the throat is perfect?

OP - I'm in the wait line.

Wait. Try to chamber empty resized case. If your die is improperly adjusted, you may not be able to close the bolt. If you loaded improperly sized cases, you get to pull bullets and maybe trash the brass you malformed.
First three steps to reloading is 1) read manual 2) reread manual 3) reread manual.

There's about 5+ items I can think of that could cause a major problem if you reload the ammo ahead of time. Once you get the rifle, it will not take you long to load up some ammo. Just have the brass prepped, primed, and ready to reload. Them once you have the rifle, you can chamber check the ammo as you load it.

What is the brass you have on hand now? i it new 280 AI brass? Is it 280 brass to fireform into 280AI ?...
I am generally in the camp of wait but if you want to have some ready or almost ready to roll and want brass fireformed to your chamber then you could load some up in the starting load range and seat the bullets long so you can finish seating them when you have the rifle. '

I would probably just prep the brass to primes ready for bullet and powder then wait on the rifle. It doesn't take long to measure for bullet seating depth adjust dies then add powder and bullet to the case.

I ordered all new brass from midway. Its all Hornady 280AI brass.

Thanks, guys. I will just get the brass primed and wait to measure the chamber when I get it.