Definitely option 3. Practical experience confirms this. And no, this is not counterintuitive.

The powder charge is a fixed amount of energy in the system. With new brass, some energy is required to stretch the case leaving less for to drive the bullet. When the brass is fireformed, more energy is left over to drive the bullet resulting in higher velocity. The difference in size between the new brass and the chamber is proportional to the velocity gain.

For instance; my target rifle in .308 Win with 0.001" headspace, tight neck and small base shows a velocity gain of around 12 fps. My hunting rifle that is CIP specs shows 25 fps gain.

I have had a loose chamber or two that needed fireforming before being able to even try develop a load. Turned out that one of my earlier test loads was spot on and being used till this day, but was useless on first firing. .303 Brit BSA Mod 1
My .270 shows no difference in velocity or accuracy from first firing onwards.

I recently did some calcs with QL and GRT where a case volume difference of 1gr between two different case types, caused a speed difference of 50fps. A further 1gr of propellant was needed to get on the node with virtually the same speed. Caliber was 6mm Creed with 109gr bullet.

Oh WOW, Thanks, that is exactly the information I was looking for. You are a goldmine of information.

GRT and QL use the case volume as a measure of how large the chamber volume is when case is fully expanded. Changing the case volume does not simulate case fireforming. Using it as such, calculations will err high when using new brass volume.

There is a reason why they require the volume of a fired case before resizing.

I recently did some calcs with QL and GRT where a case volume difference of 1gr between two different case types, caused a speed difference of 50fps. A further 1gr of propellant was needed to get on the node with virtually the same speed. Caliber was 6mm Creed with 109gr bullet.

What was the difference in pressure ?

GRT and QL use the case volume as a measure of how large the chamber volume is when case is fully expanded. Changing the case volume does not simulate case fireforming. Using it as such, calculations will err high when using new brass volume.

There is a reason why they require the volume of a fired case before resizing.

Interesting
Going to run this test to see how much difference there is
I have brand new cases
Fired but FL sized cases
Going to save some fired but not re-sized cases to measure
From measuring external dimensions of a fired case I can already see an increase in size compared to an unfired / FL sized case
I am planning on having my FL size die honed so that the sizing sizes only the minimum amount necessary

I am planning on having my FL size die honed so that the sizing sizes only the minimum amount necessary

I think this is what the competition shooters are doing. They used to neck size, now they say they all full length size, but I think that is not the whole truth. When they get their custom rifle barreled, the ask the gunsmith to ream a resizing die from the same tool they used to cut the chamber of the barrel, so the size of the die is as close as identical to the chamber. They then use this to bump the shoulders back 0.001 of an inch, and they call that full length resizing.

This is very different to what we do with hunting rifles, as a full length resize can really work your brass hard if your chamber is cut with "generous tolerances" to make sure the rifle chambers all makes of ammunition. The commercial dies are probably also "generous in the other direction to make sure all resized brass fits any rifle.

If you cannot afford/ too late for a custom die/barrel matching set, then the next best thing is neck sizing of fired brass. A simple Lee collet neck size die gives good results. If the brass starts to stretch too much and you can't open the bolt, then a body size die can be used to bump the shoulders back a bit.

I am planning on having my FL size die honed so that the sizing sizes only the minimum amount necessary

I think this is what the competition shooters are doing. They used to neck size, now they say they all full length size, but I think that is not the whole truth. When they get their custom rifle barreled, the ask the gunsmith to ream a resizing die from the same tool they used to cut the chamber of the barrel, so the size of the die is as close as identical to the chamber. They then use this to bump the shoulders back 0.001 of an inch, and they call that full length resizing.

Not quite
Using the same reamer that cut the chamber you can then make a SEATING die
There is a special SIZING reamer that the reamer maker will make for you at the same time
It has to be smaller than the chamber reamer ie. it has to cut less metal in the die body so when cases are sized they come out ( slightly ) smaller than the chamber. So that they can be chambered
If you look at SAAMi and CIP you will see dimensions for the chamber and the cartridge ( with tolerances )

Cutting sizing dies using the chamber reamer have been in use since people have been making wildcat cartridges. Basically you are making a neck size bushing bump die; i.e. bumping the shoulder. Sure, you would not be using it for hunting, but it has been the acceptable practice in benchrest for a considerable time. Hell, Chris van Niekerk made his dies like that for the Tombie.

My 243 die was cut with the Ackley reamer so that it is in effect a bump die as Janfred mentions above. I have not even used it as the normal 243 Lee Collet die works perfectly on the fireformed Ackley cases.

Cutting sizing dies using the chamber reamer have been in use since people have been making wildcat cartridges. Basically you are making a neck size bushing bump die; i.e. bumping the shoulder. Sure, you would not be using it for hunting, but it has been the acceptable practice in benchrest for a considerable time. Hell, Chris van Niekerk made his dies like that for the Tombie.

My information was from MANSON - the actual reamer manufacturers ...
They would not make an extra reamer - just for extra business ?

Obviously benchrest is different - you are making a match min-spec reamer
Relying on small tolerances / brass springback to be able to chamber cartridges
I wonder what happens when the chamber is dirty - or - very hot ?
or
You try and use cartridges reloaded and sized in someone else’s  (SAAMI) FL sizing die

How many people have extraction problems ?

Well, if a person is stupid enough to use other people's reloads in his own custom chamber, he deserves what is coming to him.

The BR shooters that I know clean barrels after every string (7-10 shots), so dirty chambers are never an issue for them.

Manson and PT&G will make a reamer to any measurement you specify. I have not met any shooter yet that thought it worthwhile to purchase a reamer to make a special sizing die.

Precision shooters all use custom reamers to cut chambers. None of the profesional shooters that I know reload their own as they are usually sponsored with match ammunition using custom chambers adapted to the ammunition. The local shooters that I know that do reload use Wilson FL bushing dies or Redding Type-S FL bushing dies with Forster a distant 3rd. Nobody of any renown use Hornady dies.

The only competitors that I know that use standard SAAMI chambers are hunters with off-the-shelf rifles and UK Bisley shooters. As store-bought ammo must be to SAAMI standard. UK bisley shooters use issued ammunition in competition that are loaded to CIP standards. They do not reload for competition.

aaaaand, yet my Oupa shot springbuck in the head at 300 yards from off a running horse using a old .303 and what ever ammo he had.
? ? / how did he do that with no special chambers then ?