I have just received back my new 280ai. Due to the brass shortages, I was unable to locate any of the Nosler Custom, or even any brand of regular 280, I have instead necked down some Lapua 30/06.
The first rounds were a fairly tight fit in the chamber, which I didn�t think would be an issue for the fireforming. After the first firings, I found they had fireformed well, and that it was very difficult to reseat a projectile by hand. On measuring the outside diameter of the fired round, it is only half a thou less than the loaded round.
I assume the chamber is tight, not allowing for an expansion in this area. Before I start neck turning, how much neck clearance should I be looking for?

Proper way to do it is to know the chamber neck dimension first. Either find out from the smith what that is, or do a chamber cast to find out for sure. Measure with micrometers, not calipers.

Them measure your loaded rounds, and turn accordingly. I'd go no tighter than 1 1/2 thou a side, three thou total on a hunting gun.

Many thanks Aalf.

I am waiting to hear back from the smith. My thought was to turn off 2 thou a side, so will just need to double check the actual dimensions with the gunsmith.

Also, if I turn the necks of the un-fireformed brass, is there any danger in turning a little off what may end up forming part of the shoulder?

you may have enough clearance on the neck already as stated you need to know what the neck size is first and your brass may contract a thou or two after firing

ANY amount of clearance that allows even minimum expansion, is enough, even .0005". If you are careful and will be sure to check the necks of all ammo before firing, I'd leave it as tight as possible for help in the accuracy department. .002" overall diameter of the necks is more than enough, 001" a side.

You don't need a chamber cast. Take a case, size and prime, use about 12-13g of a fast pistol powder, plug it with toilet paper, and fire. That should give you the neck diameter of chamber, plus about .0005" to allow for springback.

No problem, as it'll actually help a tad from a donut forming on the inside of the neck/shoulder juncture.

This

I always turn my necks to leave .002-.003" clearance. I always figured it was an accident waiting to happen if getting any tighter. I take a fired case and figure my chamber neck diameter to be .001 more. .0005 could be right as mentioned above. I do have 3 rifles chambered with tight necks and have always measured the fired cases with calibers. I guess I should mic them some time

I agree with .002-.003

Having a "tight neck" is not necessarily a bad thing. As long as the cartridge will feed reliably, there is nothing wrong with the necks not being loose on a bullet after the cartridge has been fired.
Many varmint hunters specifically make it so their cartridges are that way.
If there is enough tension left in the cartridge neck after firing, you may be able to actually get away with not sizing the brass, an simply clean, de-prime, Prime, charge with powder and seat bullet.

This may not work for every situation though.
If I were you I would turn the necks so you have 80% clean up on most of you cases and call it good. don't remove any more than you have to.